I always assumed that the traction tires wear first, fronts in our case (I may be wrong as I never needed to checkup on this).
If everything's wearing the same, and driving seems fine, and the tire's are wearing evenly I personally wouldn't worry too much. Means you just don't need tire rotation.
Maybe check and see if you can figure whether you think the fronts are wearing less than they should, or the rears are wearing more. That will give you an idea of which end of the car is different at least.
