I just cleaned two Graco Argos seats. One was in my friend's car and she lets her kids eat and drink anything in their seats so it was much dirtier than my seats where I only let the kids have water to drink and rarely anything to eat so my seats weren't as dirty. There were a lot of crumbs in the bottom of the seat from my friend's car, under the cover. I vacuumed out as much of the crumbs as I could, then used a damp cloth to wipe the shell down all over. There were crumbs stuck in tiny little crevices so I poured a little hot water in those crevices and used a toothbrush to clean the crumbs out. I don't know that I'd feel comfortable hosing it off.
As for the cover, if there were obvious stains I sprayed them with Shout or whatever laundry stain remover I had, then just machine washed them on the delicate cycle with Tide. My new washer has a dedicated dispenser for Oxi Clean that dispenses it at a certain point during the wash cycle, so I used that too. I don't use fabric softener but rather vinegar instead in the fabric softener dispenser. The covers came out nice and clean.