"Back in the day" when I started quilting, polyester batting was all the rage, but it bearded when quilted.....that is, tiny bits of the batting occasionally pulled through seams and stitches like tiny tufts of beard. So, one way that could be 'somewhat' eliminated was by pressing seams closed, hence the press-to-the-dark dictate.
Today's batts, even polyester, are better made and do not beard as much as they used to, whether hand or machine quilted.
I tend to press to one side, out of habit as much as anything.
Jan in VA