Domain parking required?

Hello,

I have a few domains that i'm only using the subdomains on, the main adress (A record) I set at 127.0.0.1 because I am not broadcasting any service to the public.

Now I wonder, is this allowed? are there certain rules for this im not aware of?

personally I see no point in attaching a webserver to the domain's main A record when im only using the subdomains.

Thank you.