I would't pre-pay it because it opens the door for things to go wrong.
On NS last month, they kept adding the charges to my onboard account every day even though I showed them my booking confirmation, which stated that I had prepaid gratuities as part of a promo. I wasted a lot of time trying to sort it out with "Guest Services," who were supposedly trying to sort it out with the head office.
Long story short, it wasn't resolved before I disembarked. So I ended up using my OBC to cover it and reduced the total amount by $10-20 (can't remember), which OBC wouldn't cover. That's a big black mark against HAL because it stressed me out, wasted my time, gave me less than I paid for, and affected my holiday. I had and provided documentation of the promo. It's not my problem that there was a disconnect somewhere in their process.