I don't use a wider seam allowance when piecing backing, so I think you're fine there. I only use a wider seam allowance for rag quilts, or when the fabric weave is very loose. I assume you're cutting the selvage off the backing.
The problem is that when you quilt, it is very hard to keep the top and backing exactly straight together. This means that when you trim the quilt, you will probably be cutting off some of your blocks that are on the back. Also, it's better to make the back more than 2.5" bigger than the top. Would it be possible to put another strip of fabric next to the line of blocks, so that the top is wider? It could be a different fabric, if you're out of the one that is 43" wide.