I was raised with the left/inside lane being a passing lane. No matter what speed you're going, if you're not actively passing someone in the lanes to your right (left in some countries), you should be "cruising" in the other lane on, for example, a four lane highway. I know this isn't always possible but, at a minimum, one lane should be passing the other. When you have people just cruising right next to each other, you open up opportunities for accidents.
I typically cruise around 15 over on my daily commute, because the roads and traffic flow are conducive to that. It gets adjusted when clumps occur or when weather gets rough. At this speed, I'm normally rolling with traffic.