I trim for level flight at 1/2 throttle and leave it there....now think about this......true you will climb with power on.....but what are doing when you add power? taking off, or doing a manuver where you want altitude....can't think of many times I've added power and down elevator...nope...I don't add power on an outside loop until I'm near the bottom.....
When i have a student up I have him start at 1/2 throttle...exactly where the airplane is trimmed for.
When the power is off or reduced the airplane sinks this is exactly what I want as I'm landing or doing a manuver where I want to reduce altitude...I can use throttle to reduce the sink rate or go to a positve rate of climb on a go around with throttle....
If an airplane is trimmed for level flight at power then as you reduce power you either have to retrim or give up elevator...having alot of up elevator input on approach is not a good thing....
Bottom line is this is how I do it and my reasons.....as a student do as your instructor wants..learn his way and then after you solo you have a broader knowledge base to make your choice...
Good luck