If this question is not just about touring in The States, then it is extremely pertinent to anyone, tourer or not, when travelling abroad. While tipping in the US is commonplace, it is not necessarily the norm in other countries. For example in Mexico and Central America, it is usual to tip the kids bagging your groceries, whereas is Korea and Japan it is considered rude to leave a tip behind in eating and drinking establishments. In fact, you'll have the waitperson running after you to give you your change back.

The best advice would be to do some homework before you leave home. There are a multitude of country info websites out there to peruse for this sort of information. One such site is http://www.worldtravelguide.net/, but cross referencing is always a good idea. Another way of knowing what the local custom is, is to watch them closely when it comes to paying in a restaurant or supermarket. It's not that hard to work out what is going on through body language and gestures.