A backward swept hinge line on the vertical tail means that the center of gravity of the rudder will be lowest in the center position, so the rudder will self-center on the ground by gravity alone. Still, it is desirable to tailor the force gradient of any control surface such that there is a detent at the center position to give force-feedback to the pilot when the control surface is in its center position. This is achieved in flight sometimes by a thick or blunt trailing edge. The fixed trim tabs on the rudder of a Klemm 35 below (own work) serve the same purpose.
Mechanical detents are normally found on trottle or flap levers. Primary control surfaces are mostly free-floating, with the exception of the control column in Bombardier aircraft where there is a noticeable detent in pitch direction. Bombardier calls this the "sneeze protection".
Ailerons need mass balancing to prevent flutter. On the ground, an overbalanced aileron with differential linkage (more trailing-edge up movement than down) will always fall into the fully-deflected position due to gravity. The balancing mass on the trailing-up side will have a larger lever arm and pull the opposite side with it.
In flight the aerodynamic force will center the control surface at the trim position (which might not be the center) - that is what trim tabs or trim springs are for. But this only means that there is a zero-moment point in an otherwise smooth moment gradient over deflection angle. Some aerodynamic tricks help to increase the gradient locally at the zero point (see the Klemm rudder above), but those are not generally adopted. The smooth gradient, on the other hand, is a certification requirement.