Most of the information I've read about this says that tips do not correlate with job performance. Instead, things like race, sex, attractiveness, and other things have a bigger impact. I've even caught myself doing this, where I'll realize after the fact that the only reason I tipped extra was because I noticed the worker was either a very attractive woman/man. It's not even sexual attraction since I'm not attracted to men. My guess is that I'm more likely to notice good service if I first notice the server, which happens more when they are attractive.