As a rule, I have hardened seats installed on heads of engines that I rebuild because of todays unleaded gasoline.
Not necessary unless you work the engine really hard for extended periods. I'd heard of several problems caused by hardened seats coming loose and I've not heard of read of a single problem from not installing them. Personal choice I guess but I've put tens of thousands of miles on pre-unleaded engines, using unleaded fuel with no issues at all.
