The tip included in the bill is really only in exceptional circumstances: large parties (usually six or more) and on holidays -- Thanksgiving, Christmas, New Year's Eve -- when restaurants typically serve from a limited, price fixe menu and include a tip of 18-20% in the bill, something which is clearly stated when you make your reservation and on the menu itself.

This has never bothered me. It seems to me that if a server is willing to take the time and energy to work with a larger party (never an easy task) or to work a holiday (ugh!), then the restaurant management should set up the infrastructure to make life a little easier.