I'm going to wait and read what others have to say about this. I was taught to trim {not trainers} at full throttle but I started to trim my planes at half throttle and like it better. I trim and never bother changing after they are set. This works for me very well but my planes usually have the fuel tanks over the CG. My students trainers I just trim at half throttle and leave them there. My thinking is it's better to just leave things alone and fly the plane. I don't know anyone doing any trim work during a flight like you are asking about but I guess it happens?