Originally Posted by
Iceblossom
Even those that usually recommend other things typically recommend a 1/2" seam pressed open for your backing!
The biggest reason seems to be that the seam in the backing gets rolled up when the long-armer does their work. If it's pressed open, that's one less layer of fabric. That's also why we are taught to make the seam horizontal or on the bias so that it doesn't all go into the roll in the same spot.