I mix washed and unwashed fabrics in quilts all the time -- no problems. I really don't think it has to be such a big deal, specially if you use a color catcher in your first washes of the final product. Most of us aren't using lye soaps and beating quilts on the rocks by a river or boiling them over fires in big old pots. I think some of the rules can be tossed at will since they were put into place in different conditions. Manufacturing processes are different now too. If you're really worried, make something smallish -- a table runner or some hot pads, and see how your scraps play together before you invest more time and materials in a bigger project.