I find that many modern quilt patterns instruct to press seams open, and when the pattern calls for it, that's what I do. Pressing to the side doesn't make the seam stronger, but in a hand sewn seam, it offers some cover for the gaps in the stitching. Often there will be a design reason to press to the side, if you want one area to recede and one come forward. I like pressing to the side, as it does aid in matching the seams, and I just enjoy the look of it on the back. Things seem more solid. And of course, if you want a ditch to stitch in, you have to press to the side.