So then why do it now? Sounds like a no brainer to me. If you are worried, deal with it at a later point. I don't see any problems coming from this. Play sand is dirty so wash it, but it is not harmful. If your fish die, it's not because of the sand.
And as for it killing your filter motor, that's not a factor with just about any filter as they pull water through, not push. The motor is AFTER the filtration. If it was before, I'd understand a threat to the motor.