If you're cutting flannel from the bolt, the long ruler is invaluable! I do not prewash the flannel for this project.
If you are cutting the flannel from old clothing, I prewash! I made a baby size quilt with the cotton squares. Quilting cotton is ok but with the denim...I think the flannel seems to go better.
I will post more on the batting, or filler, for the squares on Wednesday. Until then, cut away!