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

Forgot to say, I did the Spare/Front/Front/Spare rotation about once a year. I wasn't trying as much to equalize wear as to get some use on the Spare.

Some truck tire guys told me that if you rotate an edge-worn tire to the rear duals, the worn edge should be next to the other dual, not on the outer edge of the outer or far inside edge of the inner.