I'll say not req'd.
It won't do any harm, but I don't think Ford required it.
'O' rings always have a machined groove or chamfer on one mating surface. Both my heads don't even have a machined flat around hole on brg post - just very rough casting 
I've never seen an 'o' ring in gasket set or any mention in manuals
 
Cheers Gregg

If there was one there when it came out of the Ford factory, then you should put one in. End of story