Sending it out to be machine quilted would make me think a quality job done...therefore the quilting will look great on the back of a quilt. To use a light thread on a darker background will make it show even more. Is there a reason why you couldn't use a different color thread on the backside? Does it have to match the front? I use what I think will look the best. Since my quilting skills aren't even close to what a LA can do, I try to match the thread as close as possible. But a typical LA can do amazing things and I want her quilting to show, so I want a contrasting thread on the backside.