We make samples for our kits from flannel fat quarters and we don't pre-wash them. BUT we only use Moda flannels which are "brushed cottons".
you can just open the kit or bundle of fats and begin sewing. I know this from experience. If you want to use a different manufacturer, then the story may be a different one. I believe the authors of the book have used Moda fabrics in their sample. Hope this helps!
Lisa
Quilter's Paradise, Westchester NY
www.quiltersparadiseonline.com