I found many useful tips in this topic, As far as my suggestion,
1: Wash your engine at your fourth or fifth service, not as a rutein service.
2: It all varies from engine to engine, what sort of wiring is done, so there are a lot of factors that for sure differs from car settings like for normal engine you also have to cover the plugs but at magnetic ones you dont need to...
as far as mine one, EFI and well wired, i just put a bag (I specially made it of waterproof material not ordinary plastic LEFAFA) on plugs distributor point and have insulated material on battery terminals. rest of the wiring plugs e.t.c are already installed in water proof box. so i dont care how preassured water is and where it goes....
As far as petrol or gas problem is concers, well it not problem at all for those who has got EFI, for the rest YES you have to make sure to switch your car to petrol before starting the service.