Depends. Usually I do because I trailer in the back of the back stall of my 2 horse slant when with just one horse and its just the right size that they would probably get stuck if they tried to turn around in it (when hauling one horse it is better to use the back stall as it help balance out the weight on the ball and it also provides more room for the horse). I tie so that the bottom of the tie hits the horse where the leg connects into the shoulder as I don't want them pawing and getting tangled in the rope.. When I was taking Candy on trial the owners wanted her in the first stall (bringing her to the trial place) and she wasnt tied because that's normally how she is hauled-although she was tied a couple times before and was fine. Without tying the horse may move around and unbalance the truck/trailer if the area is big enough or get stuck in a smaller area and may bother other horses but, they can lower their head as much as they want and clear their airways, not get tangled in a leadrope, have a freedom to shift around in a bigger space to choose whats the most comfortable position, and balance without lead on their head. When you tie they can get tangled in the lead (around their neck or leg), they can't lower their head all the way to snort out dust (if you tie correctly), they can't use their head to balance as much (depending on length), and can possibly get head stuck over/around divider (depending on length of tie). However, they can put a little weight on the rope to balance, but its hard for them to get up from falling down, they can't move around as much, bother other horses, etc. So, I tie loosly usually hate hate HATE when people tie so there is no loop in the rope at all!
“Good things come to those who wait… greater things come to those who get off their ass and do anything to make it happen.” - Unknown