I've actually heard this discussed quite a bit while waiting tables. While it's never been my personal experience with the restaurant industry, I have many co-workers who have said this is the case. Do you guys feel that certain races, sexes, age groups, etc... are better tippers than others?
While I may say things like "white trash" or "old people," the more I think about it... it is case specific. I have had very old people tip me well, or even go out of their way to get me a tip that they forgot to leave on a previous visit (The coolest Old Man Ever!). I have also had people that look like they just crawled out of dumpster, leave me a very generous tip too.
The restaurant is a crazy place where it is easy to get frustrated. A place where residual rage from a bad day can fester for weeks after. For me, at times... it's even had me lose faith in the human race. So, I can see where it would be easy to make a snap judgment on a bad day. However, I guess for some... it is fact (or so they think).