Had to weigh in on the question of whether a tip is completely voluntary or "required". If you look at it from the perspective of some countries/restaurants where there is "service included" - i.e., no extra tip required, because the payment for the service is included in the cost of the meal, but you might leave an extra few dollars/francs/euros for "above and beyond", then you've got to see that where service is NOT included (like most US restaurants), that means you should be "paying" for it, by adding a decent 15% in the event of normal service, and more for "above and beyond". In my opinion, absent really egregious service, the minimum tipping is compulsory, not optional.

As for chambermaids, there it is, in my opinion, completely optional, because that's all part of the hotel charges, but that shouldn't stop someone from tipping additionally where you've had good service - those ladies work super hard at a tough job for pretty small pay. I HATE housework, so I usually leave a few dollars a day in appreciation.

What I don't get, though, is the "need", especially in the US, to tip a doorman left and right for doing something as simple as summoning a taxi off the nearby taxi rank!

Just my 2 cents.