Buffet Tip?

Here's a question I've had on my mind for a while. If you go to one of those buffet types of restaurants that are popping up all over the place, do you leave a tip? Each different buffet has a different level of service. For the ones that do absolutely nothing other than grabbing dishes from tables I tend to think no. For the ones that come and at least deliver drinks to you along with getting the dishes okay some--maybe not as much as a full-serve restaurant. Then again, I really don't like the ones you have to rely on a waitress for a drink from. Sometimes they're too busy, but beyond that, what if you want a different type of drink? There's even one buffet style of restaurant you have to declare your drink when you pay at the register before hand--and they charge you seperately for the drink.

Can you tell I know too much about the buffet style of restaurants. Being on a diet I'm not likely to see the inside of those for a while now anyway.