I am a longarmer and the only thing that I request for the backing is..make it 6 inches wider and longer than the top.
I don't do sheets because they are woven tighter than batiks and the needle has a hard time piercing the fabric..this results in skipped stitches and the LAR is very short tempered...lol.
If flannel is used...I don't like it pre washed...After the quilt is quilted and bound,you wash the entire quilt and it does not shrink like you think it does.
Also..please remove all selvages from the seams when a back is pieced..actually remove all selvages from all pieced fabric. The reason is because the selvage is so tightly woven after washing and drying the fabric the salvege dries in a very hard line,distorts the quilt and makes a puckered look down the seam