I wouldn't use a power washer to clean the engine bay. Use a couple of cans of engine shine and maybe a brush to get the tough stains out and just use free-flowing water from a hose going in a "top-down" motion while rinsing.
Bag up the alternator, and any electrical plugs, etc, etc.
You can go to an autostore and buy an aerosole can of electrical component spray. This stuff cleans the electrical plugs, wires, etc etc and leaves a water resistant coated surface. Then just armoural the rubber hoses and plastic and the engine bay will look like new.
powerwashing is way overkill. simple green works amazing at breaking down most crud quickly. ive used one of those nylon dish scrubbing things with the soap in the handle and it worked good in some awkward areas to reach. what i do on the dist cap is: remove all the wires from dist cap/plugs. remove cap. remove rotor. saran wrap(or baggie) the dist and put clip cap back on over top . never had water in there yet.
remove spark plug wires and bag up the distributor securely. Other than the carb you're good to go, it don't hurt nothing to get water on electrical stuff after it dries up.
All those naysayers out there have never seen how a good steam washing will effortlessly strip away filth that a scraper wouldn't touch.