For me, piecing the back is certainly o.k. However, I advise my customers to sew the fabric with the grain all going the same direction to lessen the chance of puckers in the backing fabric as it is quilted. I just finished quilting a king size quilt that my customer had pieced the backing using 12 inch squares of fabric. She did a beautiful job of squaring the squares with the grain before sewing them together into one piece. As other customers came to pick up their quilts, they saw the back side of her quilt and complimented how pretty it was, not knowing they were looking at the back.