While I'm sure there are good waiters that do their best to treat all customers equally, I can confirm that their tends to be a snap judgement made about the tipping probability of a customer. In one place that I worked the waitstaff used the euphemism "Canadians" to refer to customers that weren't likely to tip (e.g. "I just got sat with a table of Canadians...").
That being said, I sympathize with waiters and tend to tip relatively well since I know firsthand the pressure of only making $2.15/hr + tips on an unpredictable schedule that ended anywhere from 11:30pm to 2:00am (depending on how long my last table stayed).