By shimming the thrust angle you are not trimming to have the engine push the nose down, you are trimming to no longer have it push the nose up. Once shimmed for no pitch change at any power setting it will not be forcing the nose down on takeoff, just pushing it straight. If you use elevator trim to compensate for an incorrect thrust line then you will start to have trim issues while doing maneuvers. Trimming the elevator is fast and easy, but it is always better to take the time to do it correctly.