Whatever works best for you. If you pre-pay, I'm not sure how you would go about reducing the tip for someone whose service was poor, but that rarely happens anyway and I'm sure the purser's desk would work something out if it did happen.
If the service is way about average, you can always tip extra, which is much more likely to be the case. I think I have removed tips twice in 35 cruises. And, even then, I reallocated one of them, since it was our assistant waiter. I just asked that his tip be moved to the waiter, who did everything the tho of them were supposed to do.