Today with the numbers of people with food allergies or medical issues... its not unreasonible to ask, or expect to be asked.

Depends on your reason, and how you do it.
If you have any dietary restrictions, you *need* to know what is in a dish. If you are allergic to nuts, say, better to let the host/hostess know right off the bat rather than have them have to call the ambulance later. If you are a vegetarian, best to let them know, so that they don't add bacon bits to the salad -- the one thing on the table you might otherwise have eaten.
But don't expect the host/hostess to make the entire menu conform to your requirements. Leaving out bacon bits is one thing; making duplicate dishes for everything is another. Be prepared to skip dishes and not complain. If necessary, bring your own energy bar to tide you over. The main point of a dinner party is to be social, not necessarily to eat.
Oh yes - and if you are bringing wine or a dessert, it's perfectly fair to ask about the menu so you can choose an appropriate complement.

It is poor practice not to tell the person when asking them.

If they don't eat pork, or have a serious peanut issue, asking them to a BBQ where it is a whole hog and all the food is prepared where they had fresh nuts on the same counter could be a serious issue.

I would always ask if invited to any event, what was being served and if they wanted me to bring anything

I think it's good to be honest with family and friends about real needs such as if your husband is deathly allergic to nuts and there can be none in the room, you better tell that hostess and ask her to tell people who are bringing things to avoid all nuts. But if someone's just trying to loose weight, or doesn't care for beans or something, I think they should just make the most of the situation and not bring it up.