In my home state of Illinois, at least, I've noticed on the speeding tickets I've gotten that different penalties are levied if you're going less than ten miles per hour over the speed limit than if you're going more than ten mph. I've also noticed that as long as we're within ten mph of the speed limit, no one I've ever been driving with has ever gotten pulled over.
I've also been told that it's fairly commonly known that the speedometer in any given car can be up to ten mph off, which is probably the reason for the different penalties. I imagine that traffic cops know that if you're ticketed for a less-than-ten-mph violation, you could fairly argue that your speedometer was inaccurate, and don't bother with tickets unless they have a quota to fill that month.
In other words, if you're just speeding casually, stick to about nine mph over the limit.