This. Bear in mind tipping etiquette varies hugely between country and country - here in the UK it's very much a case of what you feel like, in other countries you can be immensely frowned on if you don't follow a certain rule!
In the US, I usually try to tip between 10-15% unless the waitress/waiter was extremely good or bad. It also depends on if I use cash or credit. If I use cash, it will depend on what small bills I have and what kind of change I get back.
I used to deliver pizzas and I always made sure that if I was supposed to give someone change that was under $7, I always gave it to them in all $1's so that they could have the option of giving me $3 or $4 instead of just $2 or $5 (because I would most definately get $2 and not $5...)
I typically tip between 15 and 20 based on service. But as other's have echoed if you go to a place like France where tip is included in the bill I don't pay anything extra.
I generally tip around 20% as long as the service is good. If it's bad I'll tip a lot less based on how long it takes them to bring me a new beer, condiments or improperly delaying portions.