dont forget to swap left to rights as you do the rotation, depending on construction this may mean removing the tyres from the rims and swapping the tyres left to right on the rears..
I'd be inclined to rotate the new tyres to the front when you werent towing (or epxecting to be towing), swap them to the rears when you're expecting to do a big tow trip and change the older tyres at the same time.

