It's taken me a while to chime in. First I had to read the Freakonomics article which Runnerchick linked to, plus all of the comments!
In a nutshell, I am completely in agreement that there are 10% tippers, 15%ers, and 20%ers as discussed in the above article. What you get, assuming the service was good (timely, accurate, friendly, drinks refilled and check processed quickly) in my experience has less to do with your service provided and more to do with the person who's tipping you.
Since my mother and brother are the owners of the restaurant where I work, people often think I am also an owner (which I am not). This results in fewer or no tips from time to time, even though my parties say "everything was great!" So that gets in my way more than if I'm wearing my True Religion jeans (which I often do). The only person over the past 3 years to ever notice I am wearing TRs was one of my gay co-workers. And, as Janet says, if they want to punish me and tip less because of that, then they are usually the type who will find something wrong with my service as a way to feel ok about poor tipping. For example, people will sometimes not want to engage with me at all, i.e. respond when I ask them "how's your day going?" or something like that. Because then they connect with me and feel worse about not tipping.