At the judging lecture I attended in April, the NQA certified judge said this is what judges look for in regard to backings:
If pieced, is the piecing well done?
Is the fabric a good choice?
Does the backing fabric “go” with the fabric on the front?
If there is “back art”, is it well handled?
She also mentioned that there are no longer any “rules” about backings; that center seams are okay and piecing is okay.