I always make a point of looking to see if the service charge is included b/c as mentioned by OP there are a few places that don't and the wait staff look hassled - so I make sure to leave some dosh. But I do wonder if the staff are paid more by the hour???
I was a waitress in the summers in NYC and can remember being warned that "foreign" people don't tip from the veteran staff ... it wasn't until I dated (and married) a foreigner that I realized it's not the "norm" to tip 15 - 20% at a restaurant.
My biggest question is always with grooming - hairdressers, beauticians... etc. In the past I've always held to my tipping standard - but found that no one else does - and asking my friends who are not American - it's unusual to tip... but after 11 years abroad I'm still always sort of stuck with what to do.
In re: to taxi drivers I always round up - and tip extra if they drop me off in an area that is full of traffic and they are not going to pick up another fare right away.