I pressure washed several engine bays before, if you go to your local self serve car wash they also should have a option for engine bay soap. The water pressure used shouldn't be very high because the soap or degreaser does most of the work and the water pressure is just enough to rinse and cut away that broken up dirt and grime. If you are doing it yourself at home and your water pressure is high and cant turn it down, you want to spray in bursts and be sure to have target areas. Getting your alternator a little wet or coils shouldn't be a problem unless you really soak them down. Even in the rain they get a little wet. Take your time when cleaning engine bay but you shouldn't spend any longer than 30min to get your bay clean.

pressure washing is fine, its actually worse with distributors and carbs and stuff... FC is really trouble free.

i do like to have the engine warm for two reasons.

1. the heat helps the grease come off and 2, the water will evaporate faster