Too, it depends on how well your horses haul. If they've never done before you might not want to - I know when I first started trailering my mare she was nervous about it and if I tried to tie her she'd feel trapped and panic.
Really it all comes down to personal preference. Everyone has their reasons for tying (or not tying) and in some situations one may be better than the other. The only time I have tied is when I hauled both of my mares, who had never met each other. The filly was in the slant, and my dominant draft cross was in the box stall behind it. She kept putting her head over the slant and I was afraid she was going to start biting Yukon, who, in the slant, would have no way to defend herself or get away. So we had to tie Clem to the side... And I was panicking the whole way to the new stable