I am an advocate of the 15% automatic gratuity on parties over six. As Thomas said most of the time when the check is divided up the amount collect is the total of the check. I have been the one paying and on many occasions I have had to come up with the extra $30 or $40 just to make sure that the server was covered. I know a number of people would not do it.
On a ship, when tips can be added to the onboard account, I will give extra, in cash to the individual. I have yet to lower tips on my onboard account. On one cruise I did not tip a head waiter. He only stopped by on the last night to collect his envelope. One of my tablemates looked at him with a surprised look and said "Oh my; you are real."