Well then I guess neither of my grandmothers were "real" quilters! I never saw them fold their quilts on the bias. I have quilts made by them that are 40+ years old, that have been carried around and machine washed and been made into countless forts AND they've also survived being folded along the grain over and over again!
I think it probably makes a difference if you're making "heirloom" quilts that aren't meant to be used & that are destined to be passed down for generations (and also destined to spend a lot of time being folded up in a chest somewhere). But I make quilts to be USED, and I figure that "use" includes folding them in a practical way.