Since everyone is throwing in their tipping stories, let me give you a couple of mine.
I had an oil refinery convention I was working, and man and a woman from England sat and I served them for 21/2 hours, their bill was over $100, no complaints, no late food, was not rude, even had a conversation with them for 10 minutes about where they get to travel for work. My tip was $5.
A group of tourists from Mexico came in to drink at the restaurant i was at, a total of 10 of them. They all ordered drinks, two rounds, nothing was wrong with them, nothing was sent back, again, no complaints at all. Tip was $0 on a $90 bill.
Another one, group came in from Australia. Really nice people, had a long chat with them about how I wanted to move down there. They had dinner, drinks, dessert, there bill was well over $250, my tip was $10.
These instances everyone keeps bringing up of bellmen refusing to leave, or bartenders holding your beer , if your server is being an ass, don't leave them a tip. Ali is completely right that if the service is bad enough, just pay your bill, mention something to the manager, and leave, especially in the US. But there are alot of us out there that bust our ass to get you your food and drinks, and its a slap in the face to us when you don't tip.
To me this isnt a matter of custom, if you are doing that job, it's your responsibility to serve the customers whether they tip or not.
You are completely correct that it is our job to serve the customers, I'm only telling you that we can pick out what customers will actually give us money, and rank them in hierarchically in our heads. Look at the math:
$150 bill (a nice dinner for 2, drinks, dessert, probably lasts an hour to an hour and a half)
10%!t(MISSING)ip - $15
tip-out for the end of the night for bartenders, expo, bussers, and runners - %! (MISSING)of tips - $3
It took an hour and a half of running around, checking and re-checking on your food and drinks, seeing how you liked the food, refills, and trying to make you feel comfortable with some conversation, to make $12. That's $8/hr., the standard bartender makes AU$20/hr. - US$18/hr. They are nice to you over there because they want to keep their job, they track sales of everyone and that is how they are rated and scheduled for the week.
The "tipping culture" may not be to hot in Bali, the argument here isn't should you tip and ignorant and rude person, but if you receive good service in the US, please tip.