Butt-first, head-first, sideways... My boa's eaten her food pretty much every way you could think that a snake could (or couldn't) eat. You've got a young boa, right? When you get her on rats, there's really not a way to eat those sideways, so she'll straighten herself out eventually. I had to feed my girl multiple mice when we first got her (we had to wait a month to go to a show and buy bulk rats) and they were small enough that she just folded them in half and ate them that way if she ended up grabbing it by the middle.

You could also offer the prey head-first (using tongs, just grab the mouse/rat by the scruff of the neck so that it's facing your boa) so that the head is the first thing she grabs if you're really worried. Odds are she'll end up eating it that way if that's where she grabs it.