I refuse to wash (or let anyone else wash) my engines. Even on those rare occasions when I've had cars detailed, my instructions to the detailer were: Wipe down what you can reach with a rag, but don't you dare get in my engine bay with a pressure washer or even just a hose.
I think my visceral reaction to the concept of washing an engine comes from back in the day when cars had ignition distributors and spark plug wires. If you washed an engine down back then, you could pretty much count on it running like crap for for a while until it got good and hot and everything dried out, or you took all that stuff apart and meticulously dried it and applied some WD40.
-Uwe-