This is why I have asked this question.
We were recently on the Crown. After dinner we were given the slip to sign for the charge. There was no place for a TIP. I asked the Maitre'd after dinner and he said they are not capable of doing that and if we wanted to leave a TIP it would have to be cash. We told him we were not carrying any cash just our room sign and sail card. He then said not to worry it that everything is OK. We went back after going to our cabin to get some cash. Naturally no one is going to refuse some extra money. Other passengers on the ship told us that they never TIP them as it is included. One couple said they dined in The Crown Grill 3 times and didn't leave any as it was included. So this was not a case of never seeing the servers again as some people feel. To add more confusion to this is when you get room service you are required to sign a slip and there is a line for the TIP.