Originally Posted by grammyp
My grandmother said it was so the batting didn't poke out through the stitches. Don't know if that is the original purpose for pressing to the side, but it works for me.
Yes, that was the original purpose, but also for a stronger seam once quilted. I have some older quilts that the stitching has started coming apart, but you can't see the batting poking through because the maker pressed to the side, and I am able to whip stitch the seam back together easily.
It's personal preference. Sometimes I press open on a small item to reduce the bulk at the seams.