No airplane really needs thrustline offset to fly however adding in offsets can reduce the pilot workload. I usually start with zero down thrust and 2 degrees of right thrust. These are my standard for an aerobatic model. They will get adjusted based on a few things. My question is on your P-51 is does the wing have a positive incidence. If so what will most likely result is that the airplane will pitch trim at a certain power level but add more power and it will climb. Some consider this desirable but I do not. If your model does this and you choose to want to eliminate or reduce the tendency then some down thrust will help. Right thrust will help during take off runs. Without it the model will want to veer off to the left and you will have to compensate with right rudder. Adding right thrust will reduce this to a degree but you will still need to be rudder proficient. This and other trimming techniques are just a matter of what expectations you have for the airplane.