You do have a choice on the NEW system. Pre-pay or wait and and use your SBC once on board.

On the old system that (I think) will be in place for Viking you simply request and then you can pay with your SBC when you board.

I know people worry about getting excursions but in most cases you can get on excursions once you are on board. There may be a few limited availability excursions that sell out but for most excursions it usually is not a problem to book these on board. For example on this trip they accommodated everyone and kept advertising their excursions.

On the new system if an excursion is REALLY important to you then pre-pay. If using your shipboard credit is REALLY important to you then wait and do it on board.