Thank you Niklas.
Tipping is serious business and you need to read up on how the countries you are visiting deal with tip. I understand now, why the guy who took our luggage to our hotel room in Vegas was pissed when he didn't get a tip. We had tipped everyone else and just from arriving at the hotel till we got into our room we had almost tipped $30. We were young and confused.
Like you say, it depends on the country. If the staff rely on the tip to make ends meet, I think it is a nice thing to tip properly. In Norway, they make a decent living without tip so you are actually not obliged to tip a single cent. If the service is good, I can tip up to 10% anyways.
One thing that tip does is that it also gives you the ability to let the staff know you are not satisfied. I have left restaurants leaving $0.00 tip on purpose to let them know that this was a lousy experience. When tip is not required, you lose this option ;)