i always prewash flannel ..some woven more loosely than others ..all shrink and all shrink at different rates. i use 1/2" seam allowance whether piecing blocks or for backing. i do this w minkie, faux fur & other novelty fabrics like the minkie thpe hancoks used to sell. i've used all of the above frequently for quilts & toys for my grands ..and everything has stood up well to too frequent laundering by their mother and childrens typical rough usage.