I have a dead bar on my longarm rack so I really don't have too many issues with vertical seams. I do it all the time on my own quilts and have gotten many client quilts with vertical seams. But that is me and I have run across many longarmers who do have issue with them. Additionally, I have found that if I roll the backing back and forth a few times when loading it alleviates the saggy areas caused by those vertical seams considerably. Same for when I advance as the quilt is completed and rolled on the takeup roller. Unfortunately those who have racks with no dead bar don't seem to have this capability and it creates massive problems for them.
I have had a lot of fun with pieced backings and making quilts that are practically reversible like this one:
http://www.quiltingboard.com/picture...t-t258728.html
And this one:
http://www.quiltingboard.com/picture...t-t209647.html
But again, my LA is a real workhorse and has no problems going through multiple seams and I have heard of other longarmers that won't touch them or really dislike them due to those issues.
The John Flynn diagonal back method is great but only works for certain size quilts. Once the quilt gets over a certain size you can't use this method.
I love wide backs for ease of loading and also to save me the time and trouble of piecing a backing. And really, in the long run, wide back fabrics are cheaper, by a long shot when you do the math as compared to standard 42 - 44" wide. fabric. When given a choice for loading, I definitely have a preference for the wide backs and strongly recommend them to my clients and I have a large stash of them for me too.