I'm curious about this as well. We just bought a 3 horse slant and I have been debating on what is safer to tie, or not to tie.
A few weeks ago, a girl was driving to a show, hit black ice, the trailer came undone from the truck, and flipped over. The horse inside was uninjured except a few stitches and soreness. The owner's fully believe that the reason she survived was because she was untied. http://www.mlive.com/news/kalamazoo/...ler_jack-.html
I'm worried about my horses trying to "duck" underneath the divider if they aren't tied. So I've been debating on what to do.
I'm curious to hear more answers.