I prefer just soaking it in Purple Power and then using regular pressure from a hose.
But if there is a bunch of crud a pressure washer may be necessary. I have done it many times, never covered anything. Just don't aim at the alternator, fuse box, stuff like that.
I've never had any trouble. I just make sure to drain the battery tray after since water collects in it, and take it for a drive to bake and blow all the water and moisture away.
When I used to detail cars running the engine during the procedure was something the detail manager insisted on. Power wash away, it didn't harm the many vehicles I have done this to.