The potential problem with resetting the subscale when leaving a high FL is the possibility that you are going to be held at some intermediate level prior to transition ..
For high descent rate aircraft, leaving it until transition is a recipe for high workload entering the lower levels. However, at some convenient level approaching transition makes sense.
Going back some years, AN's practice was to set the QNH as part of the FL 150 descent checks and that seemed to work out pretty fine.