"Many pilots are told by their instructors - Fly by the trim. My instructor religiously says, No...trim after you find your appropriate angle of climb, descent, or cruise. Which is true, and which is false?" - Spencer C.
“Fly by the trim, or fly then trim. A good question. I believe both techniques can be correct. First of all, it is generally to a pilot’s advantage to have the aircraft in trim. This gives him the maximum ability to use the controls as necessary.
A pilot who is very familiar with the airplane typically has the ability to anticipate the need for trim before it is evident. In this case I see nothing wrong with trimming as the flight conditions change. For example, if I reduce the power and intend to hold altitude, I immediately anticipate the need for up trim and go directly from the throttle to the trim.
However, I do not believe that an airplane should be deliberately miss-trimmed in anticipation of a configuration or power change. I once knew a pilot who trimmed significantly aft on short final to reduce the force of the landing flare. This put him in a seriously out of trim condition for a go-around and is dangerous.
Of course we all know we have to fly the aircraft first, so sometimes it is necessary to put the aircraft where you want it and then trim later. That is normally the case on a go-around.
I hope that most CFI’s are mature enough to recognize that as long as a pilot is not using a dangerous technique, they should be allowed to exercise discretion in their trimming procedures.”