I always tie when travelling with horses.
A friend had a horse years ago that she never tied, one trip he decided to look around behind him, got his neck stuck around backwards, panicked when he got stuck, and nearly killed himself trying to get free. We had the quite literally dismantle the float to remove him, he'd gone under the divider, got a front leg hooked over the chest bar, pushed the divider through the roof so that he was completely trapped and tangled. There was no way anyone could get in there with him as he was thrashing so much.
It was horrible to watch and we all through we'd be pulling a gun out to put down a horse with a broken leg/back/neck etc.
I ALWAYS tie just short enough to stop the horse getting its head around backwards.
And if your horse pulls back, then you shouldn't be travelling with it. Teach it to give to pressure on its head - that's a ground work problem that should not be just accepted as a horse's quirk.