Sometimes. I'll tip taxi drivers by rounding up the fare mainly because they put up with a lot; driving people around all day, putting up with drunken idiots etc.
Tipping hospitality staff is another matter. As I work in hospitality I know how much the majority of waitstaff earn so it has to be for some first class service. Of course I've worked in jobs where the kitchen staff take a cut of the tips so I'm not against it.