I work at a detailing shop.... what we do is wet the whole front of the car down, engine and paint. that's to keep the degreaser from staining the paint. then we spray the whole engine down with degreaser (anykind will work). concentrating on really bad spots, and scrub them if possible. then power wash it clean. there's nothing that can be hurt by this. if your worried next time you go shopping get plastic bags and use them to covor up any worry spots. it'll keep that part dry. then once you use the power waher use a reagular hose and rinse everything off, incling the paint around the engine bay. then use a polish spray.... i can think of any off the top of my head but i'm sure you can find one.
it's that simple and if you do that every couple of months the engine will stay nice and clean.
hope that helps.