I would not say you had to use hardened valve seats unless you plan on some pretty severe use (towing, racing, etc) or doing something like running it on natural gas or propane, but I can't say somebody won't show up to tell you that you do need them. If it were me, I wouldn't worry about it.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end