I've cleaned my engine twice in the past 2 years. Covering a few components and then spraying water to rinse, then spray degreaser, then agitate and rinse, and follow up with dressing.
I recently looked in my owners manual under the care section and you should clean the engine compartment it says. Supposedly grease and crud on the engine causes the engine to heat up more than a clean one. And the hoses will last longer. It has a picture of what to cover as well and sensitive parts. But it's directions are to powerwash and use a motorcraft engine degreaser. I've sprayed water into my engine compartment without worries as it's pretty sealed off.
Keep in mind we do have late model trucks so I donno about the older ones.