I guess I'm like Marian I can't honestly remember the last time I had service bad enough to not tip or complain to the manger. I try to tip in the 15-20% range. I usually add more if I kept the server busy with special request.
I would like to pose a question. Do you tip carhops at places like Sonic? I've had people tell me yes but when I've tipped sometimes they look at me like what am supposed to do with this?