Thank you for this thread..I have been wondering about exchange blocks and this helps to be careful. I will wash all of the swaps before I sew them together.. I did not prewash fq and have had some of them not so color fast. My daughter made a blue and yellow flannel rag quilt and the blue ran into the yellow. sad, she did put new pieces over the yellow but it was so sad! I try to prewash every thing and still once in awhile something slips through..also helps in more accurate size if the shrinkage is taken care of before cutting and sewing!