(Personally i wouldnt buy a car that has had the engine jetwashed or steam cleaned......i always think the owner is trying to hide an oil leak)
So TD you like a very dirty engine and engine bay then? You dirty, evil, man
, how could you leave these engines soooooooooo dirty. Even if you have an oil leak and unable to see it, you still leave it dirty or do you clean it to find out where the oil leak is coming from? Nothing wrong wiv cleaning the engine and bay as long as you leave the engine switched off and cover most of the electrics (if you can that is) then dry it off and use damp start if it's being a pain to start. I always clean my engine all the time, but it is a 2.0ltr, so maybe the bigger engines are different, I never seem to have a problem. I suppose it all depends on how long you are rinsing the engine for, BUT never leave the engine running thats a no no.
All water based pressure cleaning (pressure washer or steam cleaning) can get water inside some of the semi waterproof seals, causing untold damage....
But surely some cleaning of the engine is perfectly ok? And surely it depends on how much pressure the presure washer is pushing out. Like I said I have had no problem's before with all the cars I have had. I don't know if the 2.5's or 3.0ltr's and bigger are different though. 
Oppps, sorry, yes, keep the engine clean

. What I was trying to say is do it by hand, not using pressured water. Personally, I would say a hose has too much pressure....
AA and Chopsdad do have impressively clean engines, I know AA just uses a cloth, not sure what Chopsdad uses....