The true "nominal" deck height of a run of the mill production 351C is 9.215 inch, unless you have a Boss 351 block, then its 9.206 inch. That's where the 9.206 dimension came from. The Boss 351 dimension is repeated in several books and even in Ford's SVO catalogs. But its in error. Just like a 351C with 11:1 compression has always been in error.
A good machine shop can level the decks, and make the two banks equal height, by shaving nominally 0.015 off the decks. That takes them down to 9.200. That's all the milling any Cleveland block I've ever seen needs to true it up. But just as often these days I've seen the decks taken down to 9.190 and even 9.180 a time or two.
I've seen piston manufacturers and crankshaft (stroker kit) manufacturers use 9.213 (Ross), 9.200, and 9.190 (Scat) for the 351C deck height. Obviously the manufacturers using 9.200 or 9.190 are assuming the decks have been milled.