Waiters/waitresses already have a pretty shitty job. On top of that, most of them rely on tips to make a decent salary. Sure, if they're assholes, then don't tip them. But when someone is busting their ass to make your experience a good one, give them a tip. My mom was a waitress when I was a kid and after all the crap she went through, it really pisses me off when someone doesn't do it.