Qunchr

I've been messing with my new APP for a couple of weeks now. I been processing 40S&W and it has work well for everything. The pain I'm having is loading the cases mouth down for bulge busting. Question I have is why mouth down? I've seen videos of machines pushing cases through rim down. I've done it both ways and see no difference in cases that come our the other side of the die. Anyone have any advanced knowledge on bulge busting?