Actually the reaction wheels should be placed close to the center of mass, not away from it. I see your logic but reaction wheels don't work like levers. To move an object with a lever the other end must be "pushed" like when using RCS. Reaction wheels, however, rotate around the part's center, not the ship's. So to get a rotation about the center of mass, the part must be at or near the ship's center of mass. To visualize this, imagine grabbing a long piece of rebar and trying to rotate it with one hand. Grabbing one end and rotating it is hard and is offcenter, but rotating the rebar at its center will preserve the position of the center of mass and make it easier to rotate.