IMO I wouldnt apply any form of chemical, pressure wash, or scrubbing to an asphalt shingle roof if I werent OK with shortening its lifespan. It might make it look better but I personally wouldnt do it but if I was absolutely set on it I would only use a chemical wash and rinse with no pressure or scrubbing, anything that can potentially remove the grit is bad the grit keeps UV rays from eating away at the asphalt.