Biggest problem I've had about washing stuff that small is that the material frays heavily. I imagine if you want to hand wash and air dry....that would work. Otherwise just sew them first and wash. The sizing will make it easier to match seams and sew straight lines.
Usually I wash everything I am working with, but in the case of prepackaged stuff like jelly rolls and charm packs I just sew first then wash to cut down on the amount of extra threads that come off of the material.
It is entirely your personal preference however!