Heard plenty of stories of coil packs being rooted by water getting in.. particularly with euros though.
If the engine cover is on, then there's no need to cover the coil packs - but if the coils are exposed, I'll always cover them with something. Same deal if the alternator is exposed. And if the car has an OTR fitted, I'll generally bag up the mouth of it even to wash the car.
Also, you don't really want to be spraying degreaser or APC on a hot motor - it will dry before it gets a chance to work - better off spraying it down when cold and letting it sit (use brushes to agitate if necessary and to get into all the tight areas), rinsing clean and hose it down - then start the car and let it idle a while to dry off. You can dress the plastics then too if you like. Also, when rinsing, I do use a pressure cleaner but keep it a bit of a distance away from the bay, so it's not copping the full pressure. You're dumping less water over the area using a pressure cleaner, but you're bound to force water in somewhere if you hold it too close.
All this said, there are some older motors that I just will not hose down - 70s ferraris spring to mind. Just not a good idea - I dry wash those. Not a problem with modern motors though. Just be wary of coil packs and alternators
I do sit on the cautious side of things as a rule though - last thing I want to do is double my business insurance by screwing up someone's motor by forcing water where it shouldn't be.
