I used to feel guilty and always tip, but not now. I only tip if the service is good and with a smile. Even abroad.
A few years back in New York, we had a rather awkward incident when we gave a tip, but the waitress told us it wasn't enough!!! She said due to weird tax laws, a tip has to be a certain amount so it can be taxed accurately, or nothing at all. As I was happy with the service, I was prepared to pay the extra 3 or 4 dollars, but my mate stood up and said " you cheeky f**kers!! Stuff it up your harris " and stormed out leaving me gob smacked.