As long as you are not spamming anyone, there are no real rules.
I have PM'd many reviewers for a little inside scoop and have shared also. Sometimes you will get some very good insight that cannot be gleaned out of the reviews.
It seems that most reviewers are willing to help with any questions as long as you are a gentleman about it.

Any particular rules about PM'ing other hobbiests, or providers for that matter, about reviews?  As a newbie, I've already discovered that I'm pretty particular.  I've had 9s that aren't my type at all and I'm starting to recognize certain reviewers as having more of my taste.