I think I'd rather let the rain do it :-)

That's my approach - we get plenty of rain. My worry's those flying bastards dumping their load all over them although after (I think it's) 4 years, they still look pretty clean to me!

If I see a big drop off in performance, I might change my mind!

Unfortunately, ours are a little less inclined than yours.

This might also mean that your roof will be easier to move around on - I find it impossible to stand up on ours so it's lucky we've got the flat roof to walk on.