I have always been confused by how we decided who to tip. When I was in high school i worked at a little store/gas station. In Oregon you can't pump your own gas, the attendant has to. I the year I worked there I got a tip once at christmas. So if someone brings you your food you tip, but not if they pump your gas. and if they bring you fast food you don't tip, unless they are bringing a pizza to your house. Its bizzare.
I still think tipping was invented by business owners so they don't have to pay a higher wage. If someone waits tables and is good they can make some decent money in tips. I had a friend who, while in college, made about 17 bucks an hour waiting tables. His first job out of college, with a degree, only paid $13.
The business owners only have to pay minimum wage and the workers make more and probably don't claim half of it on their taxes.
As for me tipping. If I go there a lot and have one bad experience, no prob. I still tip because I frequent the place. If I go somewhere and it's not a regular place I go to I will not leave a tip if the service sucked. I figure they get their wage to take the order and bring the food. The tip is for anything beyond that.