In general, it's true that tips are not obligatory. However, whether the service is good or bad, the person waiting on you is indeed01 providing you with a service. If you don't tip, you're having them wait on you virtually for free. If the service were to be so utterly appalling that you would even consider not tipping at all (or, similarly, if you are so unhappy with the service that you would be inspired to leave a truly meager tip), I think the best thing to do would be to speak with the management directly. Restaurants never benefit from poor service, and it would not be out of the ordinary for them to comp part of your meal (for example, by offering dessert or a round of drinks on the house) at that point, while handling the situation with the server in private. If one doesn't like tipping as a rule, they are always free to get take out or cook at home.
The system is set up for servers to make their income almost exclusively from their tips. As I understand it, most wait staff are paid an hourly rate that is well below minimum wage. Personally, I've never waited tables, so I don't know what it's really like to be on the working end of these situations. However, I've never really had a problem with the standard inclusion of a tip for large parties. The only thing is that I don't believe the standard is 10-15%; at least from my experience, I think it's usually 15-20%, with 18% as a common average. I do imagine part of the reason this has become a regular practice in the restaurant industry is because enough servers have gotten stiffed because of different factors that can be involved with large groups. In my opinion, a "large party" should begin at 10 people, not at 6 as some restaurants practice.
I think 15-20% is an appropriate tip in general, and I've never tipped less than that. The only question that comes to my mind about requiring a 20% group fee is this: let's say a table is made up of 12 people. If those 12 people were to have come in separately, say in four tables of three people each, those people would be free to tip at their discretion. Therefore, I'm not sure why these same people would need to pay more because they're sitting together. I've done a little searching around to satisfy my own curiosity, and I've included some arguments in favor of this practice as held by some people in the industry. I think it is fair to at least take their positions into consideration.
One thing, nonetheless, should go without saying; if a restaurant does require a group fee, it should be made extremely clear on the bill. Unknowingly being tricked into paying a tip as high as 40% would be unforgivable and, to my mind, comparable to stealing. If a charge of 15% has been included automatically, you would not be obligated to add more for an average level of service. However, on a fixed charge of 10%, I would certainly add another 5-10%, determining that amount according to the quality of service provided.
Some opinions in favor of the practice:
Tipping Etiquette for large parties | San Francisco | Yelp
The restaurant I manage includes 15% on parties of 8 or more; Honestly, 15% is a marginal tip in the restaurant industry, but our owner refuses to impose a higher gratuity on his guests at this point in time. Luckily most parties recognize this and usually add on another 2 to 5% of the TOTAL bill (even after tax). Even if the party is $1,000... a 20% tip of $200 is acceptable. Generally when a party is as high of a dollar amount than $1,000, I suppose whoever is throwing a party that size could toss in an extra $50 (for a total tip of $250). Usually when a party reaches an amount this big, it probably requires a second server or a significant amount of help from a food runner or busser... which means you are not handing that entire tip over to just your head server. Don't forget that the server is usually also required to tip out a hostess, bartender and whoever else the restaurant suggests they should "tip out". To top it off, the server is automatically taxed 8% on the sale of your check, which dwindles the actual tip even further.
the "law" about automatic service charges (?!) - Not About Food - Chowhound
Not lame in the least. One group of 16 is likely to be at least as much hassle to a server as four tables of four, and usually a lot more so - and is likely to stay sitting and chatting longer, thus reducing the turnover - and thus the chance for more tips - for the evening. Plus on a large party, at the end of an evening (and a few drinks) things can slip in the tip department.
To use an example I've given before, say the total per person for food & drink comes to $100. Someone who normally has no trouble leaving a $20 tip on a $100 tab might look at a $1600 tab for 16 people and think (fuzzily), "Wow, $320 is a lot of money - surely $150 or $200 (or even $100) is a good tip!" So the server gets stiffed. Trust me, this happens a lot, and accounts for the popularity of mandatory service charges on large parties.
Here's what Wiki says on the subject of tipping and large parties:
Tip - Wikipedia, the free encyclopedia
Tipping is customary in restaurants having traditional table service. While the amount of a tip is at the discretion of the person receiving the service, the customary tip until the 1980s was from 10 to 15 percent of the total bill before tax, for good to excellent service, and since then has risen to 15 to 20%.[citation needed] The higher percentages are often expected by servers at more expensive establishments.
When a server has not adequately addressed issues a customer has with service, the patron sometimes speaks to management to have the problems corrected before considering reducing the tip significantly. In extreme cases of awful service, people sometimes leave no tip. Some people show displeasure by leaving a very small tip, such as one penny, though this is considered to be a personal insult rather than an standard business practice.
For large groups, such as six or more, many restaurants add a standard predetermined service charge (~18%) in lieu of the gratuity.[24] Reputable restaurants usually post their policy on a sign or the menu, or require servers to inform their patrons of such charges before they order. This charge can be verified by the customer on the bill to avoid double tipping. Customers have a right to negotiate, alter, or refuse charges which were hidden until the bill arrived[citation needed]. In the case of excellent service, a customer may include an extra tip even if the establishment has added the service charge, as the server usually has no choice regarding this policy. If service to a large party is poor, a customer may try to negotiate an alternate service charge with management.
United States: Tipping & Etiquette - TripAdvisor
Tips on tipping
How you tip is very much related to where you happen to be in the world. Where I live (Italy), waiting tables is seen as a career and not as an interim form of employment for students and aspiring actors. Waiters are paid a higher salary than they are in the United States. "tips" are automatically included in the prices of the items on the menu. If service is good, you can leave something extra on the table. If service is average or even poor, you're not obligated to leave anything extra.
Some advice on tipping around the world:
FAQ - Charges & Tipping - Crestaholidays.co.uk