The problem with letting them turn around is that they will try to do it every time, even in trailers they can't turn around in. That happened to me not long ago. I don't have my own trailer so I only trailer out with friends. Well, the friend I ride with the most has a nice roomy slant trailer that the horses can turn around it. Great. I like them going out forward because I feel they can see where they are going.
But, a couple weeks back I trailered out with a friend whose trailer has a tack room at the back, so the horse in the last stall can't turn around, he has to back out because the tack compartment takes up the turn-around space. Well, my mare who is used to turning around immediately tried to turn around in that last stall. She got her head and neck all bend around and she was stuck! I mean flat out stuck! She quit struggling and stood there in an almost folded in half position. We couldn't get in there with her because she could start thrashing again at any moment. I was picturing her dieing there and wondering how we would get her body out of the trailer. There were still two horses loaded in the front that wouldn't be able to get out.
Finally one of us got up at the window and we put a couple of lead ropes on her and I asked her to turn her head back around facing the window again and she was able to do it! I was SO HAPPY. We then backed her out.
Coming home from the ride we put her in the middle compartment in case she tried to turn around again she would have room and not get stuck. But she actually backed out willingly that time.
So now I'm afraid I have to always get her in the habit of backing out, even though I don't like it either, because there are times she might be in a trailer she HAS to back out of. So she has to be willing to do it and not always assume she can turn around.
But I really feel for her, because I can tell she is so tense and scared about backing out. She always squats when she backs out, expecting the drop. And like someone else suggested, I tell her "back, back, back, down, down, down" right before she is about to go off the edge. But I don't know if she knows what that means.