I don't prewash anything (I know, I know...I'm supposed to!) and I just finished a quilt with a cotton top and flannel back. It was a gift for my aunt and I washed it before sending it to her. No problem at all. That said, it was a nice Moda flannel and I wouldn't hesitate to do it again. If it had been a looser weave or cheaper flannel, I probably wouldn't have chanced it.

I always use flannel for backing (have for many years) I ALWAYS prewash flannel but never the top.

Yes, pre-wash the flannel. It shrinks more than regular cotton.

I love to use flannel for the backs of my quilts and I always wash and rinse it in warm water, then dry it in a medium dryer. It ALWAYS shrinks several inches. So to avoid puckers, I would recommend you wash it before you make it into anything. Flannel has a very loose weave to it so it will shrink. I always buy at least 6-9 inches more than I anticipate I will need and there have been times that was not enough.

I haven't used flannel BUT I remember a tip I received about it. Take it to the laundry mat.

I always prewash flannel due to shrinkage. Recently I made lots of the self binding baby blankets as there was a whirlwind of baby showers. I had several pieces I got at Joann's that shrank and pilled SO BAD I didn't even use them. Such cute prints though but were scratchy and faded after washing,wasted money. Others I got at Joann's were fine.

I have 7 yards of Joann's flannel in my washer right now - like most everyone else I prewash flannel - and only flannel. This will be a backing for a quilt.

Yep...I wash and dry twice....no nasty surprises that way.