Paul,
Mostly for appearance. Modern ships are designed with the vertical CG approximately lined up with the centerline of the wing cord. As Tim said the VCG effects the roll attitude of the ship. If the VCG winds up above or below you should trim the weight distribution to move it in line with the leadout tip guide exit point.
This can be done by adjusting the weight of the wheels, wheel pants, muffler (tilt up or down, full or tongue style) etc. You check this by hanging the model by the leadouts and seeing how it hangs. It is most apparent on low wing profiles (Sterling Yak, Mustang), for these models you can try to locate the leadout exit point as high as you can on the top side of the wing and use heavy wheels. The last little bit of roll can be trimmed out with a flap tweak.
Best, DennisT