If you are building a 4 (or more) engine plane and it has counter rotating props the motor torque is balanced out and if it has tricycle undercarriage the "P" factor is eliminated too. If the engines are mounted on the wings they are likely close to the CG so down thrust will have little effect so set them all zero zero. Use nose wheel steering to keep straight until the rudder takes effect.
Remember full size rarely have down and side thrust so they have to live with the fact that any power adjustment requires a retrim to maintain straight level flight. Unless of course your RC model has full telemetry and gyros then you can "set" the altitude and direction just like airliners do!