Depends what you mean by "pre-pay." We typically pay our tips on the second day of a three day cruise and add additional cash to the receipt stub before handing out the envelopes. We like avoiding that last-day line.
Once we wanted to pay the standard tips at the beginning of the cruise, with more to be added later (I was curious if this would affect service, as there was a thread debating the topic). But guest services was not set up to do it that early.
Barb