In most places I've lived, 10% of the final amount is added to the bill in a restaurant. If you go to a hairdresser, manicurist, you tip, but it isn't a percentage of the bill, just a flat amount (small), if you don't leave a tip you won't be chased down the street by a mad waiter/tress, yeap, I've seen it happened. In the US (& Canada) it's customary for you to leave a tip. It's not mandatory, but it's expected (read the last line on the paragraph above). After working in the hospitality and restaurant industry for so many years I understand why it's expected. Restaurant staff earns about $2/hr, who can live off of that?
I've learned to leave good tips. I always do, be it at the hairdressers, restaurants, bars, etc. And if someone from another country comes and visit, I always tell them to tip good. It wasn't always like that, I didn't realize how important it was, until I was managing a restaurant.
So, here is my little guide to tipping in the USA, you can print it too!
Note: I don't tip bad service, fast food joints, coffee shops where you stand in the queue to order or anywhere where there is no table service.