I was too slow this morning - and the debate was such fun - I thought it worth adding an anecdote of my own to the question of "is tipping optional" in the USA?
I was in Breckenridge a few years back, at a ski resort. Ordered soup for lunch. Waitress took ages to bring the soup, and then spilled it all over me when it did arrive. So I didn't leave a tip. She stood, in the doorway, preventing me from leaving the restaurant screaming at the top of her voice about "foreigners who don't know it's obligatory to tip in the USA" and basically giving me a lecture on the subject. Its not something I will forget.
And I didn't tip her - I hold firm to my own principles.
Tipping in the USA is an APPALLING practise. And as others noted on the thread, you get perfectly good service from waiters and waitresses in "non-tipping" nations such as Japan, Australia, NZ so there would appear to be zero evidence of it "improving" service.