It is best to clean then [ when ] you have been out in the rain as that softens up the muck held in there.
But that would be only once every three years or so ! My roadster is allergic to rain, so we avoid it like the plague.
The last time it saw rain was on the Eureka Springs trip.
I was so tired when I got home that I didn't wash it right away, it sat in the garage and cried for a whole week, too ashamed to go out in public !