I tip 20% and up at full service places, and generally 20% at buffets, unless the server is pretty much not around, then it goes down from there. I tip bartenders 10-20%, never less than $1 per drink. A lot depends on what they have to mix/pour, if they can give advice on wine, and how personable they are.
I've only stiffed one server at WDW, the guy we had in Mexico. Our meal should have been free the food and service was so bad. I have left only 5-10% to a couple of others, because they obviously didn't care to put forth any effort to give good service. I overheard one saying very nasty things about another table. That's fine, say what you want, but not where other guests can hear. I wondered what she was saying about us as well, put a damper on the meal...and her tip reflected that.
Anne