If a car's engine can not be washed with the pressure hose while being off, because of this or that, it is not worth keeping.Even in the distributor era of engines we used to hose the engine with just covering the distributor or spraying it with diesel or other off-the-shelf chemical.
But then again, an off road going vehicle gets the engine compartment full of mud and other filthy stuff you can get rid by only hosing off.
Contrary to general belief and car-wash places tactics, today's cars, because of all the sensors and electronic connections need to be washed with the engine off.If a damage is to happen it is going to be worse with the engine on instead of off.
A good practice is to always use pressurized air, when done, to get rid of excess water in all sensitive areas.Extra caution with the battery junction box and the PCM connector.With the battery junction box, water sips in through the connectors of fuses and relays and after some time problems arise like the fuel pump not working, etc.
In my tool box there is always a small bottle with diesel fuel in it.It is a life saver when you are stuck in the middle of nowhere because of electric problem.
PS.:Everything above does not refer to detailing the engine compartment to look shinny.