Hardened valve seats - do they really matter?

So i am about to invest a couple hundred bucks and mill a slant six head .100

However I remember in auto school being told that the 73 and up heads have hardened valve seats to run better on the then new unleaded fuel.

I can say I have never run a lead additive on the 73 and older heads and I've never had any issues.

Before I put money into machine work on a head like that, should I try to find a 73 or later head to mill, or just mill my 68 head and don't worry about it?