Prepay tips, I don't think so. That's tantamount to going to a restaurant & tipping the wait staff before you've even ordered.
My wife & I pay handsomely for the level of serice that we require. Well inexcess of the lines expectations.I realize that this approach does not put us within the great majority of passengers. So be it!
We expect & pay for service to attain the level of our expectations.Seems pretty simply on the face of it.
In our seven cruises, we've never encountered any problems whatsoever.
Best of luck with whichever approach you embrace.