A smoothly running rocket engine will have a center of thrust axis. If the engine is gimbaled, that axis should pass through each of the gimbal axes (if it's a classical gimbal), or if it is a ball joint, the gimbal point.
I am sure engines are fabricated carefully to be symmetrical to high tolerances, but nothing is perfect. Any misalignment would result in a static torque, which would have to then be maintained by whatever mechanical system was actuating the gimbal motion, and if there is a mechanical disadvantage such as the one in the way these images suggest, the job is even worse.
So do they test fire engines and monitor the position of its center of thrust with respect to the gimbaling joints, and shim or adjust the engine mounting or nozzle in order to zero out any major asymmetries, or do manufacturing processes result in engines that are naturally very very well centered?
Here is some text from previous comments that help further define why centering might be important:
If the thrust axis does not pass through the center of rotation of the gimbaling motion, the engine would have a tendency to "auto-slam" to one side as soon as it starts, so the actuators would have to deliver substantial force quickly, and track the moment-by-moment increase in thrust as the engine starts. This could potentially be much faster than the rotation of the entire spacecraft since it involves only the moment of inertia of the individual engine, not the spacecraft.
The actuator would have to continue to deliver this extra force against the engine's thrust for the entire time the engine is running, (perhaps more of a problem for hot, battery-draining electric actuators than for hydraulic actuators) then track the thrust of the engine back to zero as it shuts down. From a dynamics and control system point of view, is a different animal than steering the spacecraft.