Hi all,
I generally always use muslin for backing, but would not use flannel sheets or blankets for batting. First it is harder to quilt through if you are hand quilting, the quilt it heavier and most that I've seen, have a tendency to ball up and wrinkle heavily when washed and it shows. Muslin is easier to quilt when hand quilting. I have several that I made 20 years ago and though the cotton prints have faded and some of the stitching has wore out, the muslin is still like new.
Personal taste.