Hi,
Regarding trimming... the aircraft should be in a trimmed state for all phases of flight... and by that I will simplify it to Climb, Cruise, Descent, Approach. No trimming in turns.
I dont teach my students to trim before 300ft QFE on the climbout unless the the aircraft is a long way out of trim. Then.. hand back on the throttle until at circuit height.
Trimming on approach is a must... its hard to land well without a stable approach.. if you havent got the speed stabilised and approach stabilised with the aircraft in trim by 300ft then go around and try again.