You have to be brave to do this..
I've done it a few times with excellent results. Best time to do it is before cleaning the car. Lift up the bonnet and spray this stuff everywhere (apart from around the battery, dizzy, and coil)..

(Autoglym engine & machine cleaner)
Wash the rest of the car then go back to the engine bay when done, that will of given the cleaner plenty of time to soak in. Then get the jet spray on it on the lowest setting (sort of mist) and get off all the cleaner but avoid the same areas as mentioned above. I've done it several times with excellent results. Like I said though you have to be brave as anything electrical could easily break if you're not careful. I've not had any problems with the car not starting after as of yet but make sure you have a can of WD40 and VAG COM ready just incase..