We prepaid our tips for the first time on the Legend last month.
And we also prepaid them for our upcoming cruise.
We did not see any different in service on the Legend. It simply appears on your Sail and Sign as a credit.
However, I would suggest that if you do this...make sure you take your receipt showing the prepaid tips with you.
After day three, it still was not posted on our Sail and Sign and we had to have the Purser's Desk fax our receipt to the corporate office.