I'm glad to see replies from seams-open quilters b/c I was beginning to feel like a quilting oddity. My mom taught me to sew when I was little but I'm a self-taught quilter. On the 1st few, I pressed to the side because that's what all the books said to do. I soon switched to pressing open because I like the flat flat surface it makes for quilting and I don't like seeing all those little ridges in my design. Admittedly it is more time consuming. There are places where I press to the side, like borders and sashes.
Having said all that, I will say that if you do a lot of QItD, press them to the side or there won't be a "ditch" to sew in. It's hard to stay in line w/o that ridge of fabric to follow and I wouldn't want to sew directly ON the threads of the piecing seams. I typically ignore the construction lines altogether and FMQ all over the place.
The usual argument is that pressing to the side makes the seams stronger. I have repaired a number of old hand-pieced quilts. The ones that came apart at the seams were either sewn with longer, looser stitches, not quilted as much, or maybe just loved a whole lot more. It appears that stitch quality made the difference, same as with a machine sewn quilt. I've been using (and machine washing!) my quilts for several years now and I've not had any problems with the seams giving way --yet. Time will tell, I guess.