Depends what type/spec washer you use really as most like diy electric cold water karchers etc rely on chems to help but the industrial type high spec washers will clean most surfaces ok.
The only time you may need to use chems is on stains that the pressure washer you are using at the time don't remove very well like stubborn algae stains, lichen (black spotting), oil etc..
What type of washer do you have?