It's generally recommended that you don't get any closer than 30cm. Also watch your tyres, which can be damaged if you use too narrow a jet.
If you're washing the whole car with the pressure washer, be careful not to direct the jet at any seals - these can cope with heavy rain, but 100 bar of pressure may cause them to let water in.
Finally, if you really care about you're car - they're best avoided altogether. The best way to wash a car is with a hose to loosen the dirt and remove any gritty deposits, a bucket of lukewarm water with some quality shampoo added, a non-silicone sponge, a bucket of cold water to shock the dirt out of the sponge after each usage, a synthetic chamois leather to push the majority of water off the car (not to dry it) and a 100% cotton, unsized towel to completely dry the car off.