But even within the US, where you tip, and how much you tip varies hugely - I would doubt a US citizen would be 100% 'right' (and by right I mean doing what the vast majority do, i.e. what is customary for that place) all the time.
True, but with 56 years of experience living and working in the US I am right on tipping practices much more often than I am wrong.
Now that someone has pointed out that the IRS assume 8%, it does make me wonder why people tip double that, as that is clearly what the IRS assumes to be reasonable.
No, the IRS sees it as a minimum tip, not a reasonable tip. 15% is the normal tip for decent service in a restaurant, 20% for excellent service.
Also, remember, that in some establishments the tips are pooled and shared by the entire service staff. That is all "behind the scenes"; the customer has no need to know what happens to the tip after leaving. It could be the server actually gets 8%, and the other 7% are shared with the cooks, bus people, food runners, etc.
I agree fully with everyone who finds the system to be confusing. I would LOVE to see things change to a non-tipping culture. However, I live and work in a culture where tips are customary and expected in certain establishments. I tip accordingly, as I respect the dignity of honest work. Stiffing the tip, in situations where is it customary and expected, is not respecting the dignity of honest work. It's asking someone to work for you for free. That's just my opinion, and I'm sticking with it.
If others want to change the system by not tipping service workers, that's their business. Be prepared for lousy service when you return, though, and be prepared for lousy service if word gets out that a certain subset of people (nationality? eg) are known for stiffing the tip. I have known restaurants that actually refuse service to notorious lousy tippers.