"Hosing down" an engine won't do much good, but every few years I clean my engine compartment with Gunk. I remove the belly pan and air cleaner housing and seal the air flow meter inlet and master cylinder cap vent hole with duct tape. The Gunk should be sprayed on a hot engine, allowed to soak in for a few minutes (use an old paint brush or toothbrush as required to work any accumulated areas of grease in the nooks and crannies) then hose it off with a light spray from a garden hose.
The Gunk will dissolve grease and form a water soluble solution, which will be rinsed away with the garden hose spray. Follwing this I use soap and water to wash the surrounding engine compartment areas using old paint brushes/toothbrushes to get into all the nooks and crannies.
Following the final rinse use a leaf blower to blow away standing water, then install the air cleaner, start the engine and drive the car for at least a half hour to evaporate any water that has gotten into electrical connectors. If the engine is reluctant to start the distributor cap must be removed and dried, but normally it will start up.
This procedure will not cause any harm if done properly, but the post rinse drive is very important to ensure that all water evaporates espcially any that seeps into electrical connectors, and it's best to do the job in warm/dry weather.
Certain areas of the engine tend to get an oil film, so in between gunkings I just use mineral spririts in a spray bottle to wash away any small oil film accumulations.