Ross, there is very little chance of movement anyway, when you use a hardened set-screw because the crankshaft is soft steel, and the set screw bites into it. Because you have a key as well, rotational movement of the shaft is impossible; the set screw is only there to prevent axial movement. The axial slippage your mower suffered at some time in the past must have happened when both set-screws were loose, which is a matter of operator error, not a design fault. It is fairly common practice with setscrew-retained shafts to dimple the shaft to give a specific axial location you can return to each time. If you decide to do this, keep the dimple shallow: depth to be no more than half the diameter of the set screw, and diameter to be the same as the set screw. Remember, though, if you do that you cannot use axial adjustment of the coupling position on the shaft: you will only be able to use the screw adjustment on the clutch. Personally I would not dimple the shaft; I would make a shallow circumferential scratch around the shaft on the engine side of the coupling, to show the correct location.
