Even so, I object to the concept of tipping for such service. I find the tradition morally repugnant and insulting. It is burdensome to me and patronizing to the server. Just raise the prices of the food by 20%, pay your staff adequately and leave me out of your company's personnel issues. I am there to eat food, not to assess & reward your employees' performance. If I wanted to do that, I'd apply for a managerial position instead of showing up as a customer.
The difference is that you have a much more personal relationship with the server than you have, say, with the clerk at the local supermarket or drugstore.
Having worked in a restaurant, I can tell you that it’s not easy to get the food out hot and on time, and it’s very easy to fudge a customer’s order. There’s a lot of pressure and sometimes you have to beg the chef to get your order out promptly. Depends on where you work.
If you get good service, tip well (because tips are shared and everybody, not only the server, benefits). If you don’t, don’t tip but make it clear why you didn’t. This may not be the fault of the server, but letting management know about this will also put pressure on the other people in the chain (ranging from the chef to the busboy).