Tipping in America is hardly discretionary, but mostly expected. When you eat in a restaurant you will find your local tax added and it is up to you to add a service charge which is expected to be around 15% to 20% of the total before tax.
Other places you may find it a good idea to leave a tip are in a bar, taxi, hotels, valet parking, hairdressers - you get the picture. And at Christmas, everyone is tipped - mailmen, newspaper men, garbage men, anyone who does you a service year round.