While all of this discussion of the gromet and screws is great, it should not be overlooked that the cutout for the servo must be large enough that the servo does not make contact with the wood on the perimeter.

Also, when placing a servo in a control surface application, the long axis of the servo need to line up with the direction of force. This allows the gromets to be held with the minimum of force, while doing the best job of preventing the rocking around of the servo when pushing or pulling.