Just read an article on Yahoo that many people in US think tipping is annoying and should be banned. Their argument is that they already paid for the service and no need to give extra money. Yeah right. Server only get paid $2.5 an hour vs. the minimum wages (I think the minimum wages is $7.5 and varies by states), restaurant owners will be forced to pay them double the wages and you don't think restaurant owners will pass the cost to consumers.
I personally think tipping is a good way to communicate with your servers. The rule of thumb is tipping is not an obligation, it is a token of appreciation. The better service you get and the more gratitude you should pay. I usually tip siervers 20% of total bill (food + tax) if I think the service is above average. if I go to a high-end restaurant, I usually 25% or more. If I feel the service is average, I leave 15% tips. I rarely leave 10% for tips unless I feel a bit pissy. I never 打鐵 which means no tips at all. If you have worked for a Chinese restaurant before, you know what 打鐵 means
You should also tip the people who takes of you routinely. on Christmas time. An extra payment for Christmas is nice and that is what I usually do for my clean lady or hair stylist.
I know people don't leave tips in European countries, nor in Asian countries. I usually tip them some; I guess I must be a nice guy.