Tire rotation with duals

We don't wear our tires out. They get replaced do to age but they have most of their tread left and no signs of abnormal wear so no need to rotate. We used to rotate all 6, our tire dealer does it for free as a part of the care he provides for purchasing tires from him, but we stopped rotating when I realized it really wasn't necessary.
In the 2015 ford F250-550 owners manual Ford recommends rotating side to side front and back for DRWs and to not break up the dually pairs.