For the last ten years or so -- maybe longer, as I think about it -- 18-20% has been pretty much the norm. A lot of places will automatically add 18% for larger (6+) groups. An ex of mine, a career food server/bartender, pointed out that, among her colleagues, less than 18% was viewed as either cheap or uninformed and anything under 15% was downright insulting. Sometimes the insult is deserved. She also pointed out that lots of servers get paid minimum wage and really depend on the tips for a living. Oh yeah, this is in Portland, Oregon.