Professional Longarm Quilter Dani F. recommends this method of piecing your quilt backs:
If the quilt is too big for a single width of fabric, consider piecing it in thirds. This makes a better looking finished back than simply running a seam up the center.
For a quilt that will be longarm quilted, be sure to allow 4″ of extra backing fabric for EACH side of the quilt.
So, How much fabric will I need? Dani provides a diagram and some rules to go by when purchasing fabric for your quilt backing. This includes the 4″ of additional fabric on all sides. (Dani’s Rules)
Seam both sides of fabric to form a long tube. Use a walking foot and pin the seam so that the pattern lines up the entire length of the seam. Identify one of the original fabric folds, or the center of one half of the tube. Snip at this point and rip the fabric along this line. This will place your 2 sewn seams roughly in thirds across the back.
If you do choose to have a center seam:
Select the selvage edge without the white stripe for your center seam – the selvage that is printed all the way to the edge. Fold the entire length of fabric in half with the selvages on opposite sides. Sew the designated side together using a generous 1″ seam allowance. Sew to within 6″ of the fold. Snip about 4″ along your fold and RIP IT!!!