I always leave 20% plus. I never get horrible service, really can't remember the last time. I leave at least 20%, and usually a few dollars on top of that, or more if the service is great, or if the meal was really cheap. I used to save fortune cookie fortunes in my wallet, and leave the good ones with the tip if the server was great. One time I had one that said "You will inherit a large sum of money," and left it on top of the pile.