I have never had any problems with cleaning up the engines on any of my cars. Yes, when I was a kid and I was doing my dad's 1971 Mercedes, I would cover the distributor cap, but now, I don't think twice about washing the engine.

In fact, every used car that you purchase in dealership has had the engine shampooed/steam-cleaned as part of its detailing. As a buyer, I personally hate it, for the fact that now any signs of leaks/oil sweat areas/etc. are no longer there for me to see.

When I do it, I normally heat up the engine to its hot operating temperature, spray the cleaner on, and rinse it off. I don't go out of my way to flood the electrical parts with hose... I just kinda quickly spray/rinse with lower pressure water and move on. I also wash the plastic covers/etc. with soapy water too, just like I would wash the exterior of the car.