Alright I'm going to be different here and say why tip a set percentage?
First off I'll say that I do normally tip, whatever I feel is appropriate based on service and never a percentage.
Sometimes, there's no tip at all.
Best thing to ever come out of a Quinten Tarantino film was Steve Buscemi's rant on tipping and he makes a valid point.
I've had people say to me that waitresses make minimum wage and do hard work, well I worked a minimum wage job for 3 years and it wasn't one that society has deemed tip-worthy and on top of that it involved cleaning out deep friers, serving customers food and being on my feet for 9-12 hours a day.
Then I work as a waiter for a few months at a place that does tip sharing and make nothing on tips even if I wowed customers (and I often did) with my service, the owners would take the tips, take a cut and share them out...so I quit that as you can well imagine.
And apparently waiters and waitresses work long hours, tedious work for little pay..I got news for you, I work long hours, doing tedious work, for very little pay nowdays so why should I take that money to a restaurant and tip someone to pay for their rent when no ones tipping me jackshit to do my job which is harder than slinging food for 12 hours a day.
If I'm tipping it's at a topless bar where the food is cooked by Jesus Christ himself and the service is instantaneous..
Well perhaps not quite that good, but I don't just automatically tip 15% of the bill.