This is one I can't answer from personal experience. I would think at least some of it would depend on your skills. Red work, which is just typically chain stitches and sturdy knots was used in things that were extensively washed from quilts to aprons and tea towels.
With me and my prewashing habits, I would probably test a piece of the cloth to see how it reacts to washing. I'd do a bit of stitching on it to see how that reacts as well. I think most flosses today are pretty shrink/color stable but again, I run tests on products I don't know well. I would think if it holds up well as in doesn't ravel too much or shrink, it is probably easier to work on the unwashed cloth, and wait for the washing later.