In my humble opinion if you are going 55 or 60 in heavy Interstate traffic areas and you have a line of semis waiting to pass; you owe it to them to at least pick it up to their minimum or 65 mph just out of common courtesy. Now admittedly most of the time the traffic is light enough to just let them pass. However I have been caught in rush hour traffic.
Likewise I have been driving the speed limit on secondary roads and noticed a line of cars behind me. Again, I pull over and let them pass. It is the nice thing to do.
Also I do not like being in a line of cars waiting to pass a slow driver and I do not expect that other drivers do either.