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.