It depends on the jurisdiction. On the back of the ticket you receive, there is usually a schedule of fines for various traffic stops. Some jurisdiction's fine schedules for speeding don't start until 5mph or 10mph over the speed limit.
And that doesn't even take into account the discretion of each individual officer. Some don't care if you're doing 9mph over, some will cite you for 1mph over. It just depends on how much of a stickler for the letter of the law they want to be, which often goes hand-in-hand with how large their department is.
Fact is, if you're going 1mph over the posted limit, you are technically breaking the law. How much you choose to push it is entirely up to you, and the consequences of that choice will always rest solely on your shoulders
In response to that story, though, I think it's somewhat annoying that people don't get pulled over more frequently for riding in the left lane at or below the speed limit. When interstate highways and freeways were originally conceived, the left lane was intended to be a lane for emergency personnel to circumvent traffic quickly when needed. Using it as a passing lane for normal commuter traffic was purely a secondhand byproduct.
But because our driver's education system is a joke, no one has a clue what the rules of the road are anymore.