As a LRQ I can tell you what I have run into... Almost all quilts come with some sort of pieced backing... If one seam in the backing it should be horizonal... because the seam should lay on the roller ... evenly distributing the backing.. I know there are lots of ways to piece a back and one says to take the width of the fabric and run it lengthwize down the fabrid and take the 2nd and cut in half and place on both side of the 1st piece.. problem for LRQ is the 2 seams build up on the backing roller and you really have to watch the backing for a dip in the fabric. It would actually be better to make the bigger squares , I think you said 17" for all over the back at least the seams would be distributed all over and not in just 2 areas as said above. Sheets.... I haven't had a problem with them but if it were my quilt I'd want a very good quality... and wash them first to see if they really are straight.. that seem to be the biggest problem. I'd rather see my clients go to the fabric store and purchase the sale fabric at 50% off for backing than use a sheet but I also don't discourage them ... I want them to do what they want.... I'm always looking in the sale fabric for backing....
My 2 cents.
judy