Well, it can be complicated – witness the large number of threads and posts about it on boards such as this one (how much? when? when not? what if?); it's not the maths of it that is difficult.

Tipping is an idiotic practice, imho. However, that said, I do generally tip when in the US and other places with a "tipping culture", albeit through gritted teeth (although I don't think that the people I am tipping are aware of that; at least, I hope not).

It would of course be more sensible if people doing jobs such as waiting in a restaurant were paid a living wage and didn't have to rely on the largesse of the customers in some relic of mediaeval serf preferment (or alternatively rely on customers' fear of the "consequences" if they don't tip) – and it would make the pricing structure of restaurants much more transparent too – but it is hardly the fault of the individual waiters that they are in this position.

However, given that the situation exists, it becomes even more of a parody if pressure is put on the customer to tip, or to tip a certain amount, regardless of the level of service. That removes the whole point of it. On the customer's side, there's no point either in tipping on the quality of the food, which is generally outside the waiter's control. (Although why a waiter is deserving of a tip when a cook isn't...)