well - it was the opposite where I was from. I worked in a restaurant through college (*gasp - a dub that went to college). A group of witnesses came in from another town, they were driving back from a zone talk . Anyway, my country doesn't have the tradition of tipping and to be honest - the wage I was on was enough to live (at the time we made 10€ an hour base + whatever tips were given it usually averaged an extra 15-20€ a night).
Anyway, they didn't know I was a witness but i vaguely recognised one of them from being the guy that adjusted the microphones at assemblies and a couple of others as well - they were a group of witnesses, they looked too smartly dressed, though ties were off, for a sunday evening crowd. They came in and asked if there was a chance of some tables. They didn't request a big group table but split the group between 3 or 4 tables. They were polite, had their fair share of drinks and were pretty loud, but no more so than other tables.
The left after their deserts, split the bills evenly on some tables, others i could tell were pay for your own (which when some have a steak, the others a salad is pretty normal). Aside from the bill - which for each table averaged 120€ or so, on one table was a 30€ tip and on one of them - a 50€. On the night i saw that each witness had tipped 10€.
My point is, you can't generalise on diners - jerks come in all sizes
As ex witnesses, or ones on the way out...i think there are an awful lot of other things to keep our grey matter occupied than tips....