Is it bad to always plug in a laptop when a wall outlet is available? Someone told me that you should always use your battery and only plug it in when the battery needs to be re-charged. I have always done just the opposite. I always plug in if it is available and only use my battery if power isn't available.

Depends on the type of battery. With most new machines with litium batteries, it doesn't matter. I have 2 laptops, that I also plug in as long as power is available. I haven't seen any degradation in the amount of time available on battery, when I have to run untethered.