wings,
Here is the way I usually approach this. When building the plane or ARF, take the time to make sure the vertical fin is EXACTLY 90 degrees to the stab, and of course the stab needs to be level with the wing. You also need to make sure that the vertical fin is as perfectly straight with the fuse that is humanly possible. Most ARF's need some tweaking in both of these areas before gluing them in. If the fin lines up straight with the fuse, set the rudder at 0 and leave it. Balance the plane laterally and then trim any banking tendencies with aileron. On the other hand, if the vertical fin is noticeably cocked to one side, you may have to trim the rudder a bit in the opposite direction.