it depends how exactly your doing it.
i go to a carwash i used to work at, and use their "tire and engine cleaner" because i know it actually has solvents in it... does a half assed job, but youve got to repeat the cleaning regularily if theres built up dirt.
it comes out at a lower pressure, and a fairly solid stream, so you can aim it pretty well.
i just avoid all the electrical stuff i can, if i happen to spray something, i either wipe it off, or spray it down with wd-40 (let the car COOL before starting it!)
hmm, ive also been known to put it on soap, and high pressure hose everything, from crawling under the car all the way up to the insulation.
it does get it fairly clean, quickly, but youll miss spots and the electronics are in danger, so do that at your own risk.