Depends which country you are in.
I feel very put out that it is virtually mandatory to tip in America when the service is beyond poor at times.
In the UK, I always tip 10% if the service has been of what I consider a reasonable standard. I will double to treble it for service that has really made my experience at their restaurant.
If service is substandard then I will not tip at all. I have worked in the industry as an assistant restaurant manager and my standards are very high, but I'm also sympathetic to the hours and hard work put in by GOOD WAITERS.