Having lived in the United States I understand the complications and confusion involved in tipping. The simple solution to this is to ask an American. However I almost guarantee that you wont get a straight answer. It's something that doesn't really get talked about in the U.S, it just happens. The reason it does happen is because the minium wage in most areas of the United States in around $8 per hour. Tipping therefore increase the salary.

When you are planing a trip to the U.S its important to remember tipping. This is something that needs to be factored into a budget as tipping can become quite expensive.