There is no difference in strength or durability regardless if pressed open or to one side. There are still only two pieces of fabric being joined, and being pulled from the same angle. The seam allowances are just floating either way and not adding anything to to the seam. You may not notice the seams as much if pressed to the side because when ironed the edge is not as crisp and it can hide stitches, but not strengthen them. It adds bulk, but no strength.