This is the golden rule in my opinion for one simple reason: PR firms are mostly in the business of getting a monthly retainer and less in the business of getting you press. If they were really in the business of getting you press, they would charge per article produced with say a formula that was something like: "amount of space/time given to your story" x "angle taken" x "value of that space" x "percentage commission" = fee to PR firm. I have yet to find a PR firm that didn't hang up the phone on me after I asked them to do that. Also, journalists -- according to my friends who are journalists -- strongly prefer to talk with the business owners or someone from the company instead of with an outside consultant.