We opted for the pre-pay gratuities on our last RCI cruise, and we were given envelopes with 'vouchers' printed out already with the tip amount on them for each person. So we still handed the staff an envelope on the last night, but it only had the voucher in it.
I will always do the pre-pay from now on. It was so much easier than trying to figure out how much cash to have on hand to pass out. And you can always include extra cash in the env if you feel the need.