How often or ever should the tires be rotated on a c ?

Because of my Ford E-350 chassis, I tend to wear the outter edges of the front tires after about 10,000 miles. So I rotate them with the inner rear duals as they have the same straight metal valve stem. The outter duals have the Borg u-shaped valve stem and that won't work on the front. I have done the rotation myself in the past as I have 2 jacks, 2 jack stands and an impact wrench with compressor. But since buying my last set of Michelin tires from America's tire store, they give FREE tire rotation and it's much easier. Took them up on it last December.