I've been hauling in a stock for a while, and usually don't tie.I find when left to themselves they will turn around and travel backwards, the find braking forces easier to deal with. Mind you hauling up here in the land of straight roads they don't have many bends to deal with.
I think that it is more important the quality of ride the driver gives them than the trailer set up to be honest.
Do you think it's possible they turn backwards because of wind, or is your trailer enclosed? I would think dealing with brake forces would be easier for them facing forward. Interesting.
Some people believe horses haul more comfortably in slant-loads, but I don't know of any real research on it. I think it's mostly a matter of style and design. With the widths of most trailers, I think slant loading wastes less space and gives you tack compartment in the back and a changing room or tack in front as well. Most straight loaders have stalls that are quite a bit wider than necessary, which is the wasted space I'm talking about. Slant load works best for a living quarters trailer, in my opinion, using that bit of extra space with the long wall.
My dad used to have a 6-horse straight load tongue-pull that was the length of a normal 4-horse. It held 3 horses abreast, two deep. The stalls were snug for your average Quarter Horse, but once in, they hauled well. It was just under the legal width for a trailer.
Most people I talk to say they can remove the dividers and fit an extra horse in, when necessary, without problem.