Yea, I always tip for food, normally at least 20% but I'll do more or less depending on the service like others have said. Very rarely do even need to do that, I've got pretty realistic standards for the job, and even all the stuff that's out of the wait staffs control. (Too many tables, not enough people, slow kitchen, etc)
My wife was a waitress at Olive Garden when we were in college and the money from a nice tipper was always nice even if there were also plenty of weirdos. I worked crappy retail so there were no tips, but having gone through it all I try and make it a bit easier for those who are working through that point in their life.
Other services I'm a bit on the fence about, like haircuts and such. I'll leave a tip on my haircut if you get me out fast, but wasting my time while you go do other stuff just peeves me. Just get me out in 20 minutes or less so I can go on with my day!