I am with you, Julia, Dryer sheets have been around a relatively short time. There is just no way to know if there is some chemical left behind in the dryer sheets that would someday damage, or, heaven forbid, cause the disintegration of the crazy quilt many years from now. I'm sure our foremothers had no idea that using silks that had been treated with metallic salts would be a bad thing either. Metallic salts are what cause(d) many silks to "shatter" and disintigrate in antique crazy quilts.
The other reason I would not use a dryer sheet is that I prefer the stability of a good muslin. I do a lot of heavy stitching and embellishment on my pieces. I think that good muslin, along with good basting, help blocks to lie flat.
Just my thoughts, certainly don't take it as telling anyone what to do.