I don't have my own trailer so didn't vote in the poll but when my boy is travelling I always ALWAYS tie given the choice. Have trailered him untied before when I didn't have a halter [was picked up when he went footsore on a long ride, so only had his bridle] and he scrambled like a demon.
Of course he IS a scrambler anyway but so much worse untied!
I've trailered young horses and they are tied too, always. They don't necessarily know about travelling well, and my TB rears when you stop so if she's tied real short she's less likely to injure herself. She needs more training but that's kind of hard when you don't have one to train with.
You roll your trailer you're pretty much screwed anyway, just this week there was a rollover less than 2 hours from me and the horse inside died. Even if you don't have a dead horse you have nasty injuries.
I think the rule when trailering a horse is that you can never be TOO careful. Like I say mine is a scrambler so I always warn people to drive realllyyyyyyyyyy slow around corners and accelerate and brake as gently as possible. I've trailered him with company a few times, and alone as many times, and he's marginally better with company but hugely better with the divider tied across (or gone completely).
EDIT; all my horses are trained to solid tie, so it's actually good for them to be tied. If they do set back, they come forwards off the pressure almost immediately, because they know it will only release if they give to it.