You have to place the fins matching the Aerodinamic center of the fin with the CoG of the plane; otherwise you'll get an extra moment around the yaw axis...
But then again, all the T-canalisers I have seen have been mounted behind the C of G, and their purpose is to an extent the same as these Dorsal and Ventral Fins.
The aerodynamic centre of pressure is normally not the same whether the model is flying the right way up or in knife edge. Is it not the location of the TOTAL aerodynamic centre of pressure that matters, including all the stick-on bits?
Magne