Some planes just have more coupling than others, based on the design. Using some mixing, you don't have to compromise your CG, wing incidence, etc, just so it flies straight in one condition (knife edge). Mixing for knife edge might be viewed as a crutch but all the IMAC pilots I know (including myself) have some mixing in our planes to help us fly better and more precise.

For 3D, I just leave my rudder-elev and rudder-aileron mixes on. If the plane has coupling, it's going to do it in all flight conditions, not just knife edge.