You can do it any way you want to! It's your quilt!!!! That's what so fun about making quilts...It gives you freedom of expression.
I assume you intend to do this one block at a time with the batting and flannel backing cut so when you sew it together, your seams will show and be raggy on the back.
I have done something similar to what you are talking about on the front. Just sew it down about 1/2" from the edge and clip to the sewing line. It will ravel and fray out too.
Be sure to show us the quilt after it's done!!
Ditter