One thing that I don't like to see. Almost EVERY quilt top that I receive has skimpy backing. Cheap fabric, poorly pieced, mis-matching grain or color, etc. The backing is just as important as the top. Longarm quilters appreciate a mid-tone back that compliments the top with a busy design to help mask tension variations.
I just saw a video by famous quilter, Sue McCarty, who supports this. The backs also need to be generously oversized to allow pinning to the frame leaders. The one that I'm working on now only has about 1 inch allowance. This doesn't work well, especially if the back or top isn't quite square. I rejected one back which looked like an old bed sheet. In addition, the diagonal seam piecing had a big pucker in it.
This is from a longarmer who takes as many as 6 QOVs per month.