First let me say that I love the look of pieced backing. It is a fun, economical, and visually appealing choice for backing. So it’s hard to believe there could be any downside. But there is –especially if your quilt is being done on a longarm.
The job of a longarmer is to make sure that the backing is square and rolled evenly on the rollers. Otherwise you get sags and tucks in the backing. The easiest way to achieve a tight, square back is to use a single piece of fabric. This has lead to the popularity of 108″ (and wider) backing fabrics.
When you piece the quilt’s back, you are introducing different fabrics that may not have equal amounts of stretch. You may have seams that introduce fullness into one piece of fabric. You may also have fabrics that are cut partly on the bias, which again increases the stretch in some of the backing.
If you do seam the backing, it is best to do it parallel to the longarm bars. This is because seams add additional bulk to the quilt. Pieced backings need 1/2″ seams, so you have an additional 1″ of fabric bulk for each seam. If you put the seams perpendicular to the longarm bars, you are introducing bulk in part of the quilt, which makes it impossible to roll it evenly.
One more complication … longarmers generally load the quilt with the widest sides attached to the roller. So the backing seams should generally be vertically in the backing, because the seams will end up being parallel to the rollers when the quilt is loaded. However this is not a carved-in-stone rule, as the quilt design also determines which direction the quilt will be loaded.
Some longarmers have different preferences, so check with your longarmer before you piece your back. In my opinion you can never go wrong with a wide backing!
(If you can’t see this photo, please check my blog at quiltnotes.wordpress.com.)