At one time they were pressed to one side so that the quilting on top would reinforse the hand stitching, since most of us use machines these days, though there are some that still do it by hand, the machine stitching is a lot stronger and it doesn't matter is you press to one side or open, the bulk changes so especially if you are working with smaller pieces it is easier when they are pressed open to spread the bulk more evenly. For the most part it is not personal prefference. depending on what I am working on, I do either.