I work in an LQS that has a quilting business. I don't quilt for the shop but am a beginning LA quilter on my own. Whether you use fleece/minky flannel or cotton one needs the batting for the stitches to sink into. I have quilted charity quilts with just fleece on the back and it is not enough for quality stitching. FYI an 80/20 batting is used on Tshirt quilts at the shop.
I will be making my first tshirt quilt for Gson this spring and have flannel for the back and will use 80/20 batting in the center.