There are some good points being made later on in the comments.
DB's parents were very concerned with what the local custom with tipping was. In MA, the sales tax is 5%. I told them that they could triple the tax for a 15% tip, but if the service was really good, quadruple the tax. They were really good about wanting to make sure to pay enough, as I'm sure many people would be if they knew and understood the custom. I do enjoy not having to tip so much in the UK, but I feel badly when I don't tip up to the US standards over there, just used to it I guess and consider it part of the cost of eating out.
Waiters and waitresses generally DON'T want the wage to go to minimum wage because, as pointed out in the comments, it generally comes out to more than they would have earned if we didn't "tip to make up the difference." My former roommate loved working weekends because she would make over $200 in tips on a slow night! She also told me that they don't like getting tipped on a credit card because they have to declare that tip as income, but can get away with not declaring it if it's left in cash.
I do hate when people give crappy service and expect a large tip. We went to a restaurant with my family for DB's birthday and the waitress told us that it is customary to tip 20% for orders over $100. Well, the service was awful and other people who came in later than us were out before us, our food was so slow! We were all too dumbfounded to say anything at the time, but she definitely didn't get 20%!!!!
I did feel badly about tipping for valet parking attendants at a restaurant we went to on the Cap this week. They had the sign on the way out that said their valets aren't paid a salary and the tips were their only payment. I thought that was mean they don't earn a salary, but we also didn't see the sign until we had already tipped and were on the way out. I think they should have either charged for valet parking (not really a place where you would expect valet parking) to make up the cost if that was an issue, or make it more clear.