The engine compartment is a lot more tolerant of a power wash than you might think - the eletrical connections all use weather-pak connectors so I say give 'r hell. Just use common sense like you said, don't spray up the intake. Also do not spray directly into the alternator, it's ok if it gets a tad wet but don't try to power-wash it's internals. And don't try to blast the paint right off the frame or firewall; let the chemicals do the work for you rather than trying brute force and blasting it off.

Mr. P.