It depends on the pattern. I often press all the alternating blocks and rows in opposite directions so the seams nest when they meet. However, there are plenty of times they don't and I have actually ended up with fat seams and flipped seams. I don't like the fat seams because that bump can get noticed and is hard to quilt out. If in doubt, press seams open. Just don't plan to Stitch in the Ditch when you do.