if you are concerned about bulk in the seam areas pressing them open will help keep it to a minimum. pressing to one side would add an extra layer at each seam which may or may not be a factor when quilting. i think it is still personal choice and makes more difference if you are hand quilting...you want to reduce bulk, machine quilting doesn't really make that big of a difference. for backings i almost always press the seams open, i'm a long-armer and it helps the backing to roll evenly when loaded.