There is no need to tie the horse so that the rope is taunt in the trailer. BUT it should be tight enough so that the above accidents are very unlikely to occur, by preventing the horse from turning it's head. If you horse is SO bad with pressure that it cannot be tied in a trailer, on a looped lead (again, just long enough to prevent its head from being turned right back), with the tail gate closed, then you have some issues that need to be addressed.
Obviously if you are in a situation where an unhandled or similar horse is needing to be transported, you make arrangements for that, but on a regular basis, the average, broken horse should give to pressure and be able to tie in a trailer.
If it won't tie in a trailer, how on earth will it tie anywhere else where it doesn't have a solid ramp up it's backside stopping it from running backwards?????