I admit, I fold a lot of my quilts on the bias for the same reason. However, I also fold them like you do if I want to display them and sometimes because I'm too lazy to do it any other way.
It offends me that someone would imply that you're not a real quilter based on how you fold stuff - life is too short! But, you could tell her that storing a quilt in a plastic bag is something "no real quilter" should do, either.
Janet