Some workers take extremely low wages (In desperate times, some take zero wage), just to have a chance to be employed, and make their money through tips.
Most places are OK with tips, in some countries (Like Malaysia), service charge is put in place in most places like restaurants, so a tip isn’t encouraged. In some countries, hotels even make special effort to put up a signboard telling tourists not to encourage a tipping culture.
If you’re travelling, and don’t want to offend anyone, just go to How-Much-Should-I-Tip and select your country of choice, and all your questions will be answered. (It also has a funnier – but more foul-languaged sister site which I won’t link to but the link is easily visible from the site above :))
You want to know how to treat waiters, porters, taxi drivers, and get some local tipping tips, this is the one-stop-shop for getting your head around service situations which differ from country to country. So check up those rates, and never feel embarrassed again.