Yes, I was also told to dismount in the centre of the arena. It does two things. Firstly it ensures the horse understands that in the centre of the arena its not always about doing intense stuff there. Secondly, it also helps to dismount in the centre, so the horse doesnt start to rush out the gate. If you vary also where you dismount (but always away from the gate) then you keep your horse guessing. I'm only just starting into the western/reining side of things (very much a learner) but have been to a couple of clinics and was advised of these things then.
Cheers Fiona (from NZ)