Here in Northeastern US, I tip 20%. I feel guilty if I tip less and it's easier to calculate than 15%. I read somewhere a while ago that the academics are the worst tippers and I would tend to agree. Not only are they "merit-based", therefore poor, tippers, they also hog the table for significantly longer than others, being smart and having lots of interesting things to discuss over dinner.
Many years ago I used to belong to Mensa high IQ society and once I joined a dozen or so Mensans out to dinner. After the (rather lengthy) meal, it was fascinating how many of them proceeded to calculate their bill by remembering what each of them ordered and calculating the tax and the tips. I think the woman next to me noticed that I "overtipped" and gave me a pitiful expression. "The poor thing can't multiply."
In a recent episode of
"This American Life" (episode: Allure of A Mean Friend
audio archive ) a waitresss performs an experiment to see whether the tips she gets really depends on the level of service she provides. Through an experimental method that was not entirely scientific, she reached the conclusion that tips do not depend at all on how cheerful and helpful you are to the customer. As long as some minimum service level is reached, you will get the same amount of tips.