I think society as a whole has determined 18% gratuity to be appropriate. 20-25% is considered generous. However, I'm certainly not the spokesperson for the service industry. I'm just pointing out that in looking at it's tax policies, the IRS does not see tipping as voluntary. AFAIC, a far better way to deal with poor service is to complain to management and have the server reprimanded.
Exactly, also it might not be the server's fault -- understaffed or the kitchen not getting food out in a timely manner or other problems in running the restaurant that the manager can fix if he/she is alerted there is a problem.