On the Millennium at the Olympic, the fee definetly included a gratuity. Of course you could add more. They do "team" service, so there are about 4 servers. It would be very hard to extend more to just one person. On Carnival, our director of district sales indicated that the fee for the Supper Club included the gratuity. As always, it is optional to tip an additional amount.
RCI made the point that the servers did not share in pooled tips. We tipped 15-20% of what we felt the feel would have cost in a restaurant. They seemed satisfied with no plates being hurled at us as we left. Actually, we felt the service was better than the main dining room.