Always disheartening to see this rule of thumb downvoted. In here, of all places. People really telling on themselves. If someone is tailgating you they have less stopping distance if you have to brake suddenly. Slowing down increases time to react. It's not bad behavior or road raging, it's prudent sense. It can save your neck. I learned this in driver's ed. I'd rather have a cager sniffing my asshole at 30 than at 45, and if they don't have a lane to just go around, the speed limit is probably lower anyway.

I don't recommend driving at 10 under though. As in like all the time. If you're in a safe situation you should just drive near the limit, it's usually what the road is designed for.