Originally Posted by
Cari-in-Oly
When I pre wash fabric which isn't often(usually for garment making) I sew the cut edges together with a narrow zigzag or my serger then when it's washed/dried I just cut the sewn edge off. No fraying strings left in the washer/dryer and I only lose about a half inch of the fabric cutting off the sewn edge so it's worth the effort to me. I haven't yet made a rag quilt but I've used flannel for several backings and this is also what I did with the flannel.
Cari
The difference for her doing the rag quilt, is the fraying after cutting the blocks. I also wash my flannel in advance but that only effects the outside edges of the yardage, once you cut the blocks you have fresh edges (4 per block) to worry about. But prewashing does help get the shrinkage out of the way and makes the threads a little more dense to minimize the fraying.