I used to have a pressure washer, but I haven't used it in a while. It's not needed to get the dirt off of the car- gentler means will do (foam gun, mitt, hose, etc). With the pressure washer, you are blasting the dirt *into* the paint- not a good thing if you're trying to avoid marring the car.
If you put the spray too thin, sure, it'd take the paint off, but you're probably not going to try that. If you're just using it normally, you won't take any paint off (probably).