When should we Install Rules and why?
If not already installed, as soon as possible.
Why? Devs may have made certain adjustments to the options of certain programs that may be present in your PC.
It's recommended (and installer says so) to install initial rules on ReHIPS installation. These are rules for some often used programs, both standard and 3rd party along with lists of trusted command lines and vendors. Of course you can ignore these rules and just answer the alerts manually as you go, but these rules can save you a lot of time. But you can always install these rules later if you want, it'll only add rules absent in database (if some rule is already present, it'll be skipped and left unchanged).