I will tip graciously when the service is exemplary. To me the word "tip" is an acronym for "to ensure promptness"...if the service is not prompt, then I have no qualms in not leaving a tip at all. I will try to assess the situation and determine if the bad service is due to conditions beyond the control of the server. i.e kitchen, not enough servers, etc (in these cases I may leave a reduced tip and explain the reason) otherwise, if the server is just bad, then no tip.

If the "service charge" is added to the bill and it is not expalined that such will be added for a bill with a specific number of people (i,e, 6 or more people, 15% service charge added) - then no additional tip is left.