I used a fence to train mine to sidepass. Only took 10 minutes for a few days and they were doing it in the open. I don't have the need for the fence anymore but I don't see the harm in it. Like MLS said, it takes away one more direction that they can go.
A lot of reiners teach horses to rollback by using fences, that way the horse stops and turns and has the fence as a reference to move forward in a straight line.
"The greatest pleasure of a dog is that you may make a fool of yourself with
him and not only will he not scold you, but he will make a fool of himself too."