as usual, I tend to differ from what "most" do. I rarely prewash anything as I love the feel of the new fabric. Even with flannel, I usually don't prewash. I use flannel to back most of my quilts too and haven't ever had a problem. But, I've bought cheap flannel ($1/yard) that I have prewashed. After washing it, it thickens up and is no longer cheap and flimsy. It works well too.