Put the bar up and then tie. In a trailer, or in any nervous new situation for a horse we want to not allow them to pull back, or go flying backwards and learn bad things. It takes 3-5 repetitions of something for a horse to learn it right and one bad experience for them to learn it wrong. Always make sure the trailer tie or lead rope is the first thing undone and the last thing done up in a straight haul. In an angle haul, have a friend to hold the divider for you safely while you tie the horse and don't allow him to back out while tied, if there is a head door then put him in and put the lead around his neck to keep it off the ground and tie him from the outside after securing the divider.
Yes, if you tie, butt bar first, then tie. BTW, If a really nervous horse spooks in the trailer without the butt bar up and can pull back, they can easily break even a trailer tie or lead rope and get hurt even more.