Using a jet washer at any distance isn't good for the Monza's. Even if you only have the slightest of blemishes on the alloys the pressure washer forces its way under the blemish and can lift the laquer from underneath
As for the paintwork - well if you are using a jet washer and cleaning dirt off your car then you are asking for it. The dirt on cars is abrasive in itself and using a power washer that relies upon the pressure to wash the car simply forces the abrasive dirt against the paint before pushing it along the surface and eventually washing away.
The only comparison I can draw is sandblasting where air is used in conjunction with abrasive particles to take off the top layers of floors/walls/paint - you name it. The only difference is that you are using pressurised water and the particles are already on the car.
HAND WASH ONLY if you value your cars gleaming paintwork and alloys