My two pence...
I found that the practice of course trimming initially to remove the pressure on the elevator to maintain the attitude, followed by a fine trim to correct for small changes in attitude, worked well. By doing it this way, the student will have at least got the aircraft more or less trimmed to an acceptable level prior to the fine trim taking place. I found that some students got predicated on the need to trim (overly) accurately in the approach attitude and especially at the early stages meant that airmanship, circuit planning , speed/attitude control went out the window as the work load increased. I'd personally prefer someone to be slightly out of trim but fly a safe and accurate circuit than get fixated on trimming and let the fore mentioned go out the DV panel!