Unless you're driving an OLD Ford, or using a pressure washer with like 10,000 psi or steam, just wash it. You won't hurt ANYTHING under the hood, and it'll start and drive off no problem. (What do you think happens when you drive through a hard rain?)
Actually, I'd suggest not putting the wand right up close to anything, but rinsing any mud out of the alternator would actually help it's life! Same with rinsing the crud off your battery, and from around it. Nothing built since 1985 has an intake you can even spray water into. If you have an open element filter, just don't spray it directly.
I normally spray everything with a light degreaser like Purple Power or Simple Green, let it sit for a minute or so, and then rinse it off. Even just using the soap setting on the engine first, and then coming back and rinsing before you drop the hood and finish rinsing the outside will work wonders, and your engine and underhood will thank you!!
While you're in there, don't forget to do your best to spray out the radiator! It tends to accumulate that same dirt buildup over time, and is the leading reason for a lot of vehicles overheating after splashing through puddles like a TV commercial! If you have A/C, spray out the condenser from the front, and the radiator from the back with low pressure. (Don't want to bend the fins shut, and they bend pretty easy!)
Chris