Coincidentally, I took my mowers to the car wash yesterday after it rained to get the wet grass off. Normally, I don't bother. After about 10 minutes and getting sopping wet, they looked a little cleaner and I was out about $5. IMO, it's not necessary. Every so often I'll take the hose to mine, blow off the grass, etc but I don't usually worry about it. They are lawn mowers, they are going to get dirty.