Reading these replies on down-thrust or up-thrust is very interesting. My reply here may solve some problems or may just start another line of discussion.
For a long time I was confused about the nose-up, climbing problem when you add power. In several instances I almost lost control of several airplanes that were on their maiden flight due to the nose coming up and getting into a near stall conditon. The stall condition is caused by the angle-of-attack of the wing relative to the airflow being too great thus causing the upper side of the wing to lose lift due to turbulence over the wing instead of a smooth airflow.
Anyway, getting back to the problem. I corrected the problem by adding "up-thrust" to the engine.
Think about it, the engine thrust wants to pull the airplane in a straight line in line with the thrust line.
In the case of down-thrust it wants to pull the airplane in a direction opposite from the angle of incidence of the wing thus increasing the angle of attack of the wing.
I know this goes against the grain of what some of the other repliers have written here but it did work for me.
There other considerations to the nose up problem too including aileron drooping, horizontal stab angle, wing angle and of course proper balance.