Isn't the general rule of thumb that if you are driving slower than most traffic, or on the way to turning off to another road, then you drive in the inside lane. Middle lane for moderate speed and outside lane for overtaking.

i.e. If in middle lane and decide you wanna dawdle a bit, then move to inside lane?

My understanding is that you shouldn't 'sit' in the outside lane and so prevent others overtaking cars in the middle lane.