I don't have any information for you on the flannels. But I don't understand why it is not recommended to pre-wash them. If you do pre-wash you find out if the colors bleed, and if the flannel pills. Flannels often shrink unevenly, so you also get that out of the way. The only reason I can think of not to pre-wash is that flannels do tend to ravel, which is why they're so wonderful in rag quilts.