OK, I was just looking at my trainer ARF plane and you are right, the fire wall is already 1* to the right and 1* to the left. It may not be that way with all planes but ithis one is! It is my understanding that the tweek to the right and down is to account for engine torque much like the rear tail prop on a helicopter. Don't take this as the law but I think that this is the reason.

I've got another question. I am building a Piper Cub PA-11 Special and I have never built a plane from scratch before and I was wondering if I should build the firewall like the ARF which is already on an angle or if I should build the firewall straight and mount the engine mount on the angle? Any Ideas? Thanks!