If you are careful you don't need one, IMO it's more of the 'socket/lug~sprung-clip' type mounts (with a screwdriver) that need them, where a slip while mounting could put all the pressure on one side of the core, or the block itself could act as a lever if not mounted properly, stepped up onto the socket edge for example...
With the 'four mobo hole' mounting method, or a socket mount that you tighten a central screw down on after clip is fitted to socket, I don't think it's needed at all...
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