If I'm hauling a gentle old soul like Rosie, I can just through the rope over her neck and tie it and she is fine. However, with Dusty and Sassy, I tie them up. The think it's ever so much fun to put their head over the divider going down the road.
Edit; quick story.
We were at a parade and two of my fellow team members didn't have a ride back. We had a three horse so we loaded them up and went on down the road. It was a stock combo trailer...a straight load two horse in the front with a small stall like section in the back. Rosie (my horse) was in the left slot, then Pac Man on the right with Duster in the back. When we go their, we realised Pac Man was never tied up and had turned his head around and gotten his cheek bone stuck on the divider. We quickly got Duster out then tried to figure out how to get Pac Man out. He was leaning against the full weight of the butt chain, which had Rosie's side completely tightened so she was trapped too. We had 5 people (including me) under his butt trying to lift him up off the chain just enough to release it to get him out. He moved just slightly and the chain dropped out from under him and went over his back, so when his butt jumped back up he broke the chain with his back. As soon as the chain dropped, it took less than .5 seconds for everyone to jump out of the trailer, I was quickly jerked out by one of the dads. Pac Man wasn't seriously hurt, he had a sore back and was cut up, but was fine. Rosie calmly backed out like "Oh, he's done being stupid, is it my turn?"
This is why I ALWAYS tie up stupid horses.