Ummm- can I say that in the whole scheme of things it doesn't matter? Flying isn't an exact science.
You're debating TECHNIQUE. What works for one person isn't necessarily the best for another depending on personal flying style and preference.
Personally I often trim simultaneously as I change pitch or power. Right way, wrong way, doesn't matter as long as you get the job done and your student understands what the primary concept is of the maneuver.