I used to be more or less indifferent about tying horses in the trailer. That is, until I met my Arabian, Snickers. Snickers is a very *busy* horse and likes to know what's going on around him at all times - particularly regarding what's going on behind him. He never gets nervous, but is very fidgety about things going on around his rump. It took a long time for him to become comfortable with me walking behind him without his help to move his rump over quickly while keeping an eye on me. While other horses may like a butt scratch, he tenses up like you're going to tickle him or something. I once tried to pull a tail hair out of his butt during a ride when his tail was flying like a flag - that tail clamped down tight as soon as he felt my offending hand getting anywhere near his tender parts (after that, he just had to deal with that hair stuck up his you-know-what).
When it comes to other horses, he seems very nervous about their ill-will toward his hind end. On trails, if he senses another horse coming up behind him, he will politely step to one side of the trail and look back at the horses, watch them as they pass, and then continue as if nothing had happened and no amount of prodding can convince him to just ignore the other horses and keep moving.
So what does this have to do with tying in trailers? Once, a friend of mine picked me up with her four-horse straight-load trailer. Her horse was loaded in its usual spot in the back left section. She figured that our horses would be better off staggered with each other and so had me load Snickers into the front right section. Snickers went in willingly, but then immediately realized that this horse was going to be staying right behind him. His response was to twist his long neck over the spare tire and look straight back at this stranger while tucking his tail and smooshing his body as far forward as he possibly could. Seeing that this was clearly not going to work, I backed him up and put him in the section right next to the other horse, and we tied their halters so they could see each other, but not reach each other and nip one another.
However, the ultimate example came during another ride in a friend's trailer. It was a two-horse slant-load, and her horse was in the front stall, so Snickers got lucky and received that nice little triangle section in the back. I might note that it wasn't exactly large, and Snickers' isn't particularly small, either. She didn't tie her horse, and since I was indifferent, I didn't tie Snickers, either. The horses seemed to get along just fine. Off we went to our ride. Within the first five minutes, there was a lot of sudden hoof movement, and then everyone was holding still. It sounded like they were fine, so we went ahead to our ride, which was a 40 minute drive on the freeway and down the busiest highway in Idaho (which is pretty busy, by the way). Little did we know, this was the view everyone behind us was getting:
Now, I tie in the trailer. :)