It's perfectly fine to sit in the left lane, *if* you're regularly checking behind you and moving out of the way when needed

(Disclaimer: This mostly applies to 2 lane highways in rural areas)

Of course, everyone should already be doing that, riiiight? It's a waste of energy (multiple kinds) to move over every time you need to pass someone going too slow in the righthand lane, at least when the roads are busy. I was trying to keep track on the trip I just took and it seemed like I looked in my rearview mirror every 15-30 seconds or so. I'm proactive about moving out of the way of people going even faster than me. It's silly to keep changing lanes back and forth the rest of the time.