I only prewash Batiks ,Flannel, and Swap fabric..if it is mandatory,
Flannel, especially when mixing it with cottons, or used on the back or a cotton quilt...it shrinks more than cotton.
Other than that, I wash every quilt when it is done..in case it falls apart! :roll: and because of the chemicals on the fabric from the manufacturer. I love they way they look and feel after i get them out of the dryer..