It is true that folding a quilt like a bed sheet will create permanent crease marks if the quilt is left folded for a while. (I have no idea how long it would take, but I have noticed quilts that had serious creases in them.) I think the lady was right, to a point, but she was also rude in the way she expressed herself. Also, she shouldn't have wound up with a lumpy looking mess if she truly knew how to fold her quilts. I try to store my quilts on a bed, not folded, but I found the following article, which explains how to fold on the bias and why to do so. I think I'll try it out next time I take a quilt to guild. However it seems to me that the real trick is to re-fold the quilt frequently, and in different patterns. If you're always folding it in half and half again and so on, then you're always creating the same pattern, whereas if you fold on the bias you will probably get a different fold pattern each time. I think it's that first two folds that are in danger of creating creases, and after that the folds are gentle and unlikely to crease.
http://annfahl.blogspot.com/2012/02/...re-quilts.html