Blaine, I believe Deere did put them on all head studs. If you're looking at the latest parts book (PC-304), it does state that two B369R lead washers are to be used. However, I believe that's a mis-print. An earlier parts book (Repair parts 54-R) states six of the B369R lead washers are to be used on the six 9/16" studs. And all of the parts books ever published by Deere for the model "H" show that you are to use a 264R lead washer on the 3/4" diameter Oil Lead Stud that is in the bottom center of the head - and that's inside the valve cover along with the other four smaller studs. So it seems logical that Deere intended the lead washers to be used on all the head studs.
I agree with James Borders that the purpose of the lead washers is to prevent galling. After all, the center Oil Lead Stud is exposed to oil at both ends; in the head as well as the crankcase. So why attempt to seal that stud since oil could seep forward from the crankcase anyway?