I mix washed and unwashed. HOWEVER....I never heat dry anything. I hang dry everything that gets wet enough to need it and I have not had any shrinkage issues between the fabrics! Im MY experience it is heat that shrinks the fabric during the drying process! so....there ya go, my opinion!

I don't think you will have much trouble using them with unwashed fabs in your stash. I did a jelly roll quilt and bordered it with something prewashed from my stash. Looks fine.