I would say it depends on the design I am making. If you press to one side and then the next row the opposite way, the seams nestle up and you get a good match up. But, on some blocks that intersect heavily in the middle, you have to press them open to have a nice flat center. Just my personal perference, I am sure. I have heard it taught both ways. Today's machine piecing are a good strong seam, and pressing it open is acceptable.