As far as pressing seams, people do both open and to the side for various reasons. All the mainline quilting texts, instructions, etc I've read say to press to the side and then one thing I read finally said, "When seams are pressed open, there is a greater chance for the batting to beard through between the exposed stitches." The light bulb really switched on for me with that.
I think the other reason we press to the side is that the seams pull open very easily in pressed open seams, unless the stitches are very short, or you backstitch start and end. I'd never noticed when sewing garments, but my backstitching isn't always aligned with the seam, and it does make a difference with the much smaller quilt seams.
Of course, this is for machine stitching. Hand stitching is not likely to pull apart, but the bearding problem might still apply.
The other issue to consider is the intersection of seams. If pressed aside, the way to handle bulky intersections by spinning them is well documented. For pressed open seams, you're on your own. A simple intersection can use garment rules, but there's no way it will work at the center of a block like a pinwheel where 8 points meet. Sometimes we just have to follow the "rules" and have faith that someone else did the trial and error experimentation that made the rule.