Unnecessary; it is reasonably protected from water intrusion. Just don't soak it with a heavy or high-pressure stream. Both of those comments apply to the engine as a whole.
Everything in the engine bay is generally safe to hose down with a gentle to moderate spray. Never use a pressure washer. Don't try to blast away grime with water; use a decent all-purpose cleaner like P21S, agitate with a sponge or brush and hose off. Leave all trim covers in place and avoid directing the spray underneath them. Blot dry with an old towel whatever you can reach and leave the rest to air-dry.
When you do wash the engine bay, make a habit of doing so before washing the rest of the car. Not only does this keep gunk off your freshly-washed paint, it affords drying time before you start the engine. The ignition coils are about the only components that react very badly to being soaking wet--you'll get heavy misfires until they dry out--but as they are protected by trim panels and water seals, once again you will be fine unless you go crazy with the hose.
Oh, and stay away from Simple Green. It is not friendly to aluminum and there's a lot of aluminum under a BMW's hood.
At the minimum, cover the alternator.