Another person who pieces backs and advises to go off-center when adding fabric to make a backing bigger.
As I've quilted over the years, I've started doing more piecing on my backs. Incorporating fabrics that didn't make it to the top, usually because the scale is too large, orphan blocks, etc. More recently I'm trying to use up stash and since most of my fabrics are 3 yards or less, sometimes I have to look at the back as one very large quilt block.
I do not seem capable of making a baby quilt that actually fits one width of fabric. When the tolerances have been very close but not quite enough, I have put on two narrow borders along the problem length/width. You do want the additions to show, so that usually means cutting off a narrow strip of whatever was too narrow.
So, say you are dealing with yardage and a quilt that is a full 44" wide, but your backing fabric turns out to be only 42" usable. Cut off roughly 2" and then 4" strips on either side make it 48", which isn't a huge amount of extra but usable, you can always make the edging strips wider. I'm usually using a print on my backs, the bars tend to be coordinating solids.
When you baste your quilt together, work on having it go down the center and expect that there will be some degree of off-ness in the finished product, whether that is one side is slightly wider or narrower or some tapering.