That is exactly what I have been told, no need for hardened seats. I think that mode of thinking carried over from the late 70's, 80's. when everyone was worried about putting unleaded in leaded fuel cars.
There is some circumstantial evidence that indicates it was a "campaign of misinformation instituted by the oil industry itself".
There is some value to using inserts with GM products since they use a much softer alloy of cast iron then Ford does.
Ford's alloy approaches the hardness of nodular iron if not in fact being nodular iron itself.
As Randy pointed out, Ford has experimented with using the cheaper and softer iron in production at times and it just came back to shoot them in the foot since it also effects the durability of the cast in valve guides.
NONE of the Ford cast iron heads do well with inserts. They are all too thin under the valve and even when you do them, you need to use a thinner insert to avoid from cutting too deeply into the valve pocket.
With casting variations from head to head, that is very risky unless you x-ray each and every casting at every intended cut.
Unfortunately Ford iron castings have lots of drop outs the size of a BB and often are on the inside of the casting that you can't see.
Ask the head porters about that. In the '60s many would destroy two heads for every one that they ported successfully. They would also tell you that you can hole through at any point and that a "race ported head" is just a "temporary head".