And from a comment on a wait staff blog re other patrons

Tom says:

@jl

[ . . . ] Sure, people are people, and you will find exceptions to any statistic. That’s why you should never prejudge someone. Give everyone a fair chance before you deem them a stiffer. [ . . . ] But as you said, they aren’t the only ones. Here’s a list of other groups of people that tend tip poorly:

Old ladies ($2 was a decent tip in 1960, but not anymore)
Lunch groups after church (a bible tract is not a tip)
Businesses (if you offer delivery)
Lesbians (however gay men tend to tip well)

Treat all these people with the same service you offer everyone else, but you can still mentally ready yourself for the stiff.