I regret to say this, but I actually had a car w/ washers as wheel spacers on it once . Mind you I didnt put them on, I bought the car used and when i replaced my brakes i noticed.

They were only mayybe 5mm so you couldnt see them with the wheel on.

anyway, speaking from experience.. I dont doubt its dangerous... but I drove on them for prolly 7500 miles before I did my brakes and removed them.

(for reference the vehicle was an SRT4)