The idea behind the 1/2" seams on the back is that you press the seams open rather than to one side. The purpose is to reduce bulk & make it easier to quilt. Generally, the back does not have foundation quilting like the front so those seams do not get any extra reinforcement from the quilting. As a result, the only thing holding together a seam that's pressed open is the stitching itself. If you don't have enough fabric to do 1/2" seams, I'd definitely shorten your stitch length to between 1.4 - 1.6mm (16-18 stitches per inch) so it has that added reinforcement and would probably backstitch the beginning & end of any seams that won't be enclosed when you start quilting. Also, I'd strongly recommend using a cotton thread in that case as it will swell and shrink closer to the same rate as your cotton fabric when washed, reducing the rubbing & stress on the seams.
EDIT: You can also get a tiny bit more mileage out of the WOF by leaving part or all of the selvedge edge on. I do this all the time & it works out quite nicely.
Last edited by Bree123; 06-01-2016 at 07:54 AM.