We are all in the same boat. For years in the US, we left 10% without thinking. A friend who worked as a watiress put us wise one day (let's face it if you are foreigners abroad, people automatically expect that you don't know local custom and practice and make allowances). We were less amused to be presented with a bill (in Charlston, South Carolina)in an establishment where the food and service was dreadful which read "15% gratutity NOT included. I am not making this up. We wrote back "15% Service NOT provided". I was furious! I might add that this conduct was not, from his accent anyway, local and was in stark contrast to the courtesy and friendliness everywhere else.