As a bartender (ex waitress) here in downtown San Antonio (mainly tourists) I’ve pretty much waited on everyone by now. 
Women do not generally tip as well as men, *unless[i/] they’ve been in the business or know someone who is/was. They just don’t. Period. I’ve had some women make my night, tip wise, but those women aren’t very common.
And yes, black people do have a reputation for not tipping particularly well; I’ve found this to be true more as a waitress than a bartender. One of my best tips ever was from a black gentleman who was from Philly; he made my night.
Asians…haven’t waited on enough to have any opinion.
Hispanic people are generally good tippers, in my experience, but Mexican nationals are among the world’s worst tippers. When I was waitressing, we’d fight over who had to take them unless there were enough of them to grat.
Young people and old people don’t tip as well as those in the 30/50 age group. Young people because they’re usually broke and don’t know any better, and old people because they haven’t updated the percentage to match inflation. (Back in their day, a good tip could jingle; a great many still believe this. And I can’t, in all honesty, think of a single person over 65 who’s ever given me a memorable tip. Nicest people in the world, but they just don’t tip!)
Secretaries and teachers are among the world’s worst tippers, on average. So are people from non-tipping cultures, like Europe, India, etc…and as a bartender at a tourist bar, I can tell you in all honesty that the only time they tip worth beans is when there’s an American with them who tells them to.
Uncle Sam is also a very bad tipper.
People who eat/drink on his tab are usually ten-percenters if you’re lucky. Ugh.
And finally, people tip the bartender better than they tip the waiter. Don’t really know why; I think it has something to do with the fact that the bartender controls the situation, while the server doesn’t, at least in peoples’ minds. People are also generally nicer to me behind a bar than they ever were at a table.
And everyone will inevitably surprise you, for good and bad, so I try my best not to let any assumptions on my part color my service in any way. Better to blame them for being awful tippers than to wonder if I earned the crappy tip they left me.