Many of the wires are covered with that corrugated plastic cover, blasting water through the slit that runs down the length of this cover is not a good idea as the water will just sit there. Also water into the connectors is bad, some of these cleaners are quite corrosive and will do damage over time to metal connectors and the plastic insulation on wires $$$$$$$.

I have the time so when ever it has got wet under the hood I dry it off. Even after a severe rain it is surprising how little water is under there. There are places that dust and gravel sit and I sweep it out with a 1" paint brush. Then I go over it all with a detail spray and polish it up a little.

I only wash my car on the outside two maybe three times a year but under the hood it is spotless basically.
Thats not too hard as its only done 20,000 km (12,000 Smiles)