Last year we had a 14' stock trailer, the horse I mainly used last year I didn't need to tie but I did because I put my tack up front and her right next to it. There wasn't a divider. She was the type of horse where the moment you put her in the trailer she fell asleep, I just didn't want her to nudge my tack and knock it over.
Now we have a 16'6 stock trailer with a divider, when we haul our thoroughbred we have to haul her or else she will swing in the trailer. Ya a big 16hh thoroughbred swinging you can really feel it, if you tie her she just stands there. My mare will spin in a stock trailer if you don't tie her.
We always tie our horses backwards, they prefer to face backwards than forwards. We either tie with the lead ropes in a release knot, or with trailer tyes.
I will say this, no matter what ALWAYS have good, decent matts in your trailer. I was at practice once and this family had a unbehaved horse and a trailer with a wood floor. The horse paniced, fell in the trailer and they had a smaller trailer. It got kind of stuck and even on the floor of that trailer the horse kicked out 2 windows, cut its leg, and just wouldn't settle down. Finally it settled down and smart people got their lariats and threw them on the horses legs and helped her get them underneath her so she could get up. Matts are a must!
My friend had a new horse that was fine in a trailer and this horse was her dream horse. She had tied it in the trailer, the horse spooked at something and fell. Sadly, it hung itself.