I think the main question is being overlooked here.
Is it safe to get the engine bay washed?
I think this link can help everyone having any doubt:
How to Detail an Engine Bay | eHow.com
Besides, it is up to the owner how he feels about his ride. If you like it neat and clean, inside and out, you'll not overlook engine bay.
Personally, I have been washing my car's engine regularly on 2-month basis. I cover ECU and generator with plastic shopping bags, and than use tap-flow water to soften up the grime and grease. Than, I use engine degreaser and leave it for about 2 minutes. Later, I wash off the degreaser with tap water again. Finally, I use blower to dry the engine bay, and spray engine dressing to take care of those rubber and plastic parts. That's it. This way, my car's engine bay remains clean for almost two months.
But again, there are people who wear clean shirts, but dirty vests!! Up to you, guys!