The best canted cage in terms of accuracy (Dr. Snikkas Cage) uses extremely concave wheels. It helps that the wheels were specifically designed for the cage though. Concave wheels are the way to go all around in my findings. I use the snikkas wheels in a specially designed straight ofp cage and they are more accurate than any other straight cage I have other than the FDL.
Concave would be a good idea for the non-canted wheel, but would probably fuck off the accuracy on the canted. My suggestion would be flat in the canted position and concave in the flat position.