Question about backing

How uncommon is it to have the seam on you backing not centered on the quilt? I'm thinking of making a throw-size quilt that would only be about 60 inches wide. If I put the seam in the middle I'd have about 10 inches to spare on each side but if I put it more to the right or left, I could save like 20 or so inches and possibly use the fabric for the back of a pillowcase, if I have extra fabric from the quilt top

Does the seam need to be in the middle, making the fabric on either side half and half, or can I put it more to the right or left and make it more 65/35, if that makes sense