The need or not is related to the location of the thrust line to the wing, which is a major portion of the drag on any airplane. It is also related to the amount of pitch stability trimmed into the model by the selection of where the model is balanced and the resulting decalage angle between wing and stabilizer. The more "stable" the plane is the more it tries to climb from any excess speed or adding power. And thus the more desire for some downthrust to reduce that pitching up.

Which is why a lot of seriously aerobatic models that are trimmed for pretty much neutral stability can get away with a 0-0-0 thrust, wing and tail setup and why high wing trainers might have something more like -2, +4, +2 or -4, +2, 0 setups or something along that line.

Your P51 is a low wing so that helps avoid the need for excess downthrust provided you set the CG location and wing to stabilizer angles fairly flat so it has some but limited pitch stability. That being the case you could likely get by with something like 0, +1.5, 0 and it would fly well yet still have some positive self recovery from fast dives. And you would trim the CG location to arrive at where it flies level at mid throttle at that setup.