If you are replacing just the click spring then a shim would not be necessary.
If you are replacing a riveted on click then you could use a thin steel washer cut in half and placed under the click (between the click and the plate) before you hammer in the rivet, the washer is cut in half because it needs to be removeable once you have the rivet and click secured in place. This must be done otherwise the click would be so tight when rivetted onto the plate the spring would not be capable of pushing the click back in place after being moved out by the ratchet wheel.
Paul.