With very few exceptions, all KitchenAid dough hooks should be washed by hand. The metal used is aluminium and it will react to most dishwasher solutions, powders and tablets.
Most KitchenAid mixers are supplied with 'C' shaped dough hooks coated with a white plastic. The coating is designed to be easy to wipe clean. To most people it also looks better than the burnished versions.
Coated dough hooks should not be washed in a dishwasher. The base metal is aluminium which reacts with most dishwashing powders. The reaction causes the coating to lift and it will often start to peel off. This usually happens around the area where the dough hook fits onto the mixer.
Burnished dough hooks are usually the same as the coated versions but without the coating. They have the same physical shape and must always be handwashed.
If burnished dough hooks are washed in a dishwsher they usually go black. A grease-like coating is usually formed that can only be removed by scrubbing. Scrumpled up aluminium foil can be used to help clean it up and remove the black coating.
This is the version normally supplied with the new mixers. It works well with most dough mixes. It should always be used on speed 1 (stir).
The spiral version is quite a recent addition. It is available for most mixers except tilt head versions such as the KSM150, K45 and KSM90 (these versions do not have a deep enough bowl). The spiral dough hook can be more efficient on certain dough mixes.
Mixing dough takes a lot of effort. Each time the dough hook pushes dough against the bowl the metal of the hook flexes slightly. This flexing eventually causes fatigue cracks which eventually cause the hook to break. The amount of flexing depends on how heavy the mix is, how much mix there is and how fast the dough hook is moving. Decrease the mixing speed for a longer life.