I agree that it's a personal preference, however I have a couple of reasons for pressing to the side, all of which have probably been mentioned above. First, the seam is definitely stronger. It's also much easier to get your seam intersections to come out perfectly when the seams are pressed to the side in opposite directions. Because of the little ridge created by pressing to the side, there is a definite point at which the seams lock or nest. If you pin on both sides, the intersection will be perfect. Finally, and I think most importantly, if you are stitching in the ditch when quilting, you absolutely need to press to the side, partly because then you have a definite "ditch" in which to stitch, but also because if you stitch in the ditch and the seams have been pressed open, you will basically be stitching on thread not fabric, which will eventually weaken the seam.