As someone who has worked food service before, I disagree with the entitlement of 20% concept wholeheartedly. I'll consider 15% my offer for a server doing what I expect.
I always took it as a challenge to squeeze more than 15% from a table. That was on me as a salesman, and I knew that going in. If I wasn't happy with what I was making, I worked better, took more tables, or found a new restaurant to work at.
The above said, I was taxed at 8% regardless of the rate of gratuity. If someone gave $0, I still paid for it in taxes. Also, many restaurants do claim Tip Credit against servers, and I avoided those restaurants as a server.
If a server goes above and beyond, I am more than happy to tip 30-50%. If they do the job, I'll give them 15-20%. If they do less than that, I might still give them 10-15%, but I'll also explain to the manager exactly what the server during wrong, and have gotten servers fired before.