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".