Frankly, I find it annoying when a gratuity is automatically included in the bill. That makes it a service charge and not a gratuity. It takes away my ability to tip less or not tip at all if I receive poor service. That said, the last time we were on Explorer of the Seas we tipped the 'correct' total for the week, but divided it up a little differently. While we had received wonderful service by the assistant waiter, the head waiter had been quite lazy night after night and gave us excuse after excuse why we could not have, for example, a shrimp cocktail or a piece of cake. (A waiter on a previous EOS cruise had easily been able to accommodate the meager requests.) So this time when our trip was over we tipped the head waiter significantly less than the recommended amount and added that difference to the assistant waiter's tip - who was sincerely thankful.
I think that's the way gratuities are supposed to work. We never do the automatic gratuity charge.