When I have to piece a backing horizontally I used to trim off the selvedg as carefully as I could and then sew my 1/2" seam. The results were "iffy" at best as to how straight my cutting had been. Now I leave the selvedge on until after I sew the seam - using a seam allowance that is the width of the selvedge plus 1/2". This works so much better for me! When I told my BQF (best quilty friend)
about this she informed me that she's been doing it this way for a while. So ... in case I'm not the last one on the planet to figure this out ... here's to straighter seams!
I prefer larger backings and seam vertically when I can, but ... lately they seem to all come out better horizontally. I quilt them myself on my domestic machine.