I prewash all my fabrics (except small precuts which I typically do not use). I use cotton batting (W&N). I also wash my quilts when complete for a host of reasons. Get the wonderful crinkly look all the time. I, too, believe that you will get a more crinkled look with cotton vs poly batting. I don't think our quilting fabric alone generally shrinks enough to make a significant difference overall in creating that look. Batting, imo, is the major factor.