I always pay 20% no matter what. I'm lucky enough to never run into a situation when the service was poor. The girls who I know who are around 18-20 and work in restaurants... it's not uncommon to see a $3.00 tip when the bill is $67.00 etc. Girls tell me it happens more than you think with the economy in Detroit. But the truth is... people are cheap. Don't eat out if you aren't going to give 20%. Plain and simple. You aren't a banker... you don't make decisions on the interest rate for waitresses.