I am fascinated by how many LAQ have issues with seams in the backing. Granted I have only been LAQ for about 2 years now and have done a whopping total of about 2 dozen quilts (give or take) Of those 2 dozen quilts 3 were my own and every single one of them had pieced backings. Like a double sided quilt pieced so I am not talking one or two cross seams going both horizontal, vertical, and diagonal, I am talking dozens of seams. I have included orphan blocks, new blocks, strips, piano keys, and huge chunks of fabric. Here are links to pictures of them that I included pics of the BACK as well as the front. They were actually the first two quilts I did on my LA, so maybe I was too green to know a difference but I certainly did not run into any issues. No pleats, no tucks, no nothing.
http://www.quiltingboard.com/picture...rm-t77776.html
http://www.quiltingboard.com/picture...lt-t85150.html
I just completed one a few weeks ago that had a log cabin set in barn raising pattern on the back along with a bunch of other piecing (haven't posted pics of that one yet). Again, I ran into no issues at all. I wasn't able to perfectly center them, I think that is just not possible, but I got close enough.
In addition to these quilts I have had 4 client quilts with pieced backs that I was asked to center as closely as possible. So again I had vertical and horizontal seams. No issues.
I do know that my quilt rack is a four rail system as opposed to 3 which many people have. My take up roller is set above my quilting bed a good 10 inches and my 4th roller is really just a tension roller that sits low on the quilt bed where most LAQ take up roller is. So if I do get build up from a seam it does not affect my take up.
That said, I do love getting a single wide back piece. They are quite heavenly. But I have absolutely NO problems with pieced backings and I don't care which way the seams lie.