Generally, RC engines rotate clockwise when viewed from the pilots position. (or CCW when viewed from the front when you are starting it). SO the plane tends to roll to the left due to torque and p-factor. So many RC plane designs call for one or two degrees of right thrust. It can be builtin to the firewall or the firewall can be square and you can build in the right thrust on the motor mount. Quite frankly, it doesn't really matter that much for everyday sport flying on most planes.