To keep it safe Consumer Reports says get rid of the zero degree nozzle, zero degree nozzles pose a totally unnecessary risk so if you buy a pressure washer that comes with one we suggest throwing it out.
And you might be tempted but Consumer Reports says never pressure wash your roof because it's dangerous. The kickback could cause a ladder to fall backward. Plus, you end up doing more harm than good to the roofing shingles. Instead, spray the roof down with a fifty-fifty mix of bleach and water and let any moss die on its own.
Here is what you can--you clean with a pressure washer. Wood and siding made of vinyl and fiber cement can typically hold up with pressure washing. But you should use caution with Aluminum siding. Aluminum siding can actually be easily dented so if you're going to try to pressure wash it start on the gentlest setting then work your way up.
Cement and asphalt walkways and driveways can stand up to the power. For moldy mildew use lower pressure and some suds. To tackle grease stains you'll need a finer, more targeted stream.
You can pressure wash most decks, too. Start with a lower pressure nozzle to make sure you don't etch or mark the wood, but you might not even need to. Newer composite decking actually resists staining and mildew so you probably don't even need to clean them with a pressure washer. Generally a light scrubbing will do.