Hi Karen. Welcome.
I'm not sure how Carnival operates, as I haven't sailed with them yet, but maybe will in the future. On my previous cruises, I prepaid my gratuities, and if I felt there was a staff member that deserved something extra, I gave them extra in addition to a prepaid voucher. On RCI, they give you envelopes, and if you prepaid gratuities, gave you vouchers.
An example, on my cruise in 2009, by the 3rd night of a 5 night cruise, my dining room waiter knew that I wanted decaf coffee with my dessert (I didn't even have to ask her). I gave her the suggested amount by means of a prepaid voucher, and extra cash, all in the envelope dedicated to the dining room waiter.
2007 - Majesty of the Seas
2009 - Enchantment of the Seas
2010 - Freedom of the Seas