I have always pressed to one side or the other, then decided to press the seams open on a 16-patch block as part of a patchwork quilt. Those blocks went together so much easier, laid flatter and were easier to press. I think what you need to consider is how you will quilt the finished quilt top. If you plan to stitch in the ditch, then pressing to one side or the other will make stitching in the ditch much easier to maintain. If you plan an overall quilt pattern, seams pressed open will be easier to quilt.