Waiters and waitresses are exempt from minimum wage laws in the US and get a low salary which is supposed to make up for it.The best waiters in the best restaurants make a good living from a low salary and good tips. It's wrong not to leave a tip in an American restaurant, unless it is one of the rare places that pays its employees properly.

In European countries where the wait staff is salaried, tipping isn't common but one can leave small change or a pourboire.

A good bike shop should pay its' mechanics a decent wage. If you want to give something extra, go ahead, but you shouldn't feel obliged to do so.