The only time this comes into play is when you're checking engine thrust while flying. To do this, you fly WOT in a straight line and then cut throttle to idle and watch if the plane goes up, or down.

If the plane goes down at idle, you have too much up thrust, and vs. if the plane goes up at idle, you have too much down thrust.

This may sound backwards to a lot of folks but what is happening is the engine is the main factor at WOT and the plane has a little up or down trimmed into the elevator to allow for level flight. Once the engine goes to idle, the main control factor shifts to the elevator for flight characteristics. (up and down) If down trim was used to compensate for too much up thrust, you'll see the plane descend at idle due to this down trim. The reverse is true for up trim at WOT.