Powerwashing cedar can be done, if done with care. Because you can also easly blow the wood apart with that much pressure. The trick is to start off a distancve fropm the surface and gradually move closer untill you're cleaning off what you want to clean off. A "household" duty-lower pressure washer would also be better than one of those rental commercial units, in my opinion.

As to whether or not it's necessary, I can't say, but I think it's a good idea.

A roof CAN be your "friend"...rather than "that thing you hate".

Power washing any wood siding will damage it. Cedar shingles are especially subject to damage.

Consider especially that mildew in your environment will return every few months. Power washing that often will considerably shoerten the life of the wood.

Excellence is its own reward!