Singinrock,

Incidence is for lift, getting the wing at the correct angle to the horizontal tail therefore correct for the angle of the air flowing over it.
The rotating prop produces torque and a swirling airflow over the plane. There are many opinions on the effects of these two, and the cures. In practice right thrust is the simplest and probably the most effective fix. Most models need 3 degrees, aerobatic models need more, the most I've ever seen is 7 degrees, and that looks weird but the plane flew straight!

happy landings.