In the US.....
Waiters and/or Waitresses are paid a base wage.... and make the "major" part of their income from "tips". A normal tip is around 15% give or take slightly. This is where GOOD SERVICE, GOOD FOOD, etc come in to play. I can honestly say that in my travels, when a "mandatory" surcharge is added to the bill (call it what you like) services all-the-way-around seem to suffer in general.
OK.... there will be the cheap-skates that low-ball a gratuity. On the other hand..... every great waiter I've ever had seems to do quite well for themselves. Great service - Great Gratuity....... Don't do your job..... well.... find another, or get better at what you are doing!!
I often over-tip.... why? Because I like good service, good food, good humor, and a kind personality.