From what I have learned from speaking to police officers about this subject, there are far too many variables to estabablish any "unwritten" rules. Police officers are human beings and as such, are subject to being irrational when making decisions.( irrational in the sense that the same violation may be over looked this time, but get the hammer dropped on the next) If you happen to pass a newbie officer going 5 over, chances are that you will get pulled over. If you happen to pass an older officer having a shitty day, then you are likely to get pulled over. If that same older officer is in a good mood, you can probably keep on truckin smartly down the road.
So, put your chips on the pass line, roll the dice, and takes your chances.
I used to be an over the road truck driver and have found that 5 over is generally overlooked. The only time in over 5 yrs driving OTR that I was pulled over for a speed violation was in Washington state for 6 over the limit. Fortunately the trooper was on older officer and reasonable. Once he realized that I wasn't just being an ass he gave me the obligatory warning and I went on my way.