I think ironing to one side rather than open makes (not sure how to describe it) a "tighter" seam. If a seam is ironed open and stretched tight, there can almost be a gap between the two pieces of fabric, but if the seam is pressed to one side, it is "sealed" up. Also, there is the stitch-in-the-ditch issue. There is no "ditch" when you press a seam open. I also like the accuracy gained by butting seams (pressed in opposite directions) against each other which you do not have if you press a seam open. I have pressed select seams open on a few quilts I've made, but it is vary rare. But as they say, there are no quilt police. Whatever works for you.