For those that say to tie because they've known of horses that were not tied and came out of the trailer while it was in motion....well, I've also known those who were tied, and still came out of the trailer while it was in motion.
You'd be surprised at how far out of the trailer a horse can come even when he has his head tied--and when that happens he probably isn't going to be able to get back into the trailer, and he isn't going to be able to come out & get free until something breaks--and a horse going down the road half in half out of the trailer is not going to end up in good shape!
I generally do not tie when hauling.