I have several rag quilts. I have only used batting in one, only because some of the squares were already sandwiched (a UFO found at the thrift shop).
I have used 1 layer of flannel and one layer of denim on 2-3 quilts. The denim was from old jeans and I have fun with seam and pocket placement. A denim backed quilt is very heavy and warm.
I have also used just 2 layers of flannel for baby quilts. I like the look and weight.
When you use just two layers of flannel or 2 layers of flannel (I did one that way too), you do not have to stitch the layers together with an X.