Three things here...1.yes fab does have a "stretchy" side....WOF will give more than LOF...that's why many patterns/teachers/designers suggest cutting borders from LOF...less chance of wavy borders......
2. If when the selvage edge is not cut,ripped, from backing fab, when joined together that seam is very tight and can cause sag when putting on frame, and worse, when that quilt is washed can pucker that seam.....if one of my clients brings in a back that has seamed back with selvage edge still on, if I can, without destroying the integrity of that seam, I will strip it off, if not I will make small cuts into it at intervals just to release some of that tautness.
3. When I put a quilt on the frame I will make sure it is mounted with those seams going horizontal...if it's a patchwork back....there seems to be no problem as it moves pretty much as does the top.
On a few rare occasions I have had a saggy bottom no matter what, so in that case rather than drive myself nuts as I advance the quilt sandwich, I have found by putting some chunks of wadded up batting against the roller from the underside will take up that sag on the sandwich as I stitch it, then remove those "chunks" and proceed on......just what I do......
Last edited by Geri B; 07-02-2015 at 06:48 AM.