I agree with you in the sense that mine is like yours--only dusty. There are some folks who take their truck off-road and you wouldn't believe the places that mud can get thrown! I can see the need for a deep spray down in those cases.

 

I wash the underside of my truck after I go offroad. I've never found the need to wash the engine compartment, even after spending the day in mud. Yeah, I know there are electrical connections underneath the truck that are getting washed, but they've been getting wet since the truck was new and if any of them were less than perfect, they would have been found and fixed during the warranty period.

 

If not too greasy I spray down the entire cold engine compartment with degreaser or just soap suds after covering the alternator with a plastic bag. Use a soft scrub brush on various areas. Rinse off with the garden hose. Spray plastic and hoses with 303 Protectant and leave it alone, uncover alternator, shut the hood and run the engine at least 15 minutes or go for a drive.