The ships accounting systems are all computerized. All your monetary transactions (including prepaid tips) are linked to your cabin number - so your cabin steward knows even before you board that tips have been paid. Your table in the dining room is assigned by cabin (not name) and once again the wait staff knows your cabin has pre-paid. That is one reason they keep track of guests who request seating changes in the dining room, just so the right wait staff is given credit for pre-paid gratuities. There is no expectation for you to tip more, however, you may get onboard and find that someon has been exceptionally attentive, or provided extra services and you may wish to reward them.