I'm with Andy on both subjects.
I have seen valve seat recession, but ironically enough, it was on a head that already had the hardened inserts installed!
Several studies have shown that being run previously on leaded fuel seems to provide lasting protection in all but the most extreme cases.
There is also considerable doubt that most of the "lead substitutes" on the market are actually effective. There was a group in the UK (BMIHT as I recall) that did a test some years ago, and found that none of the products was 100% effective, and only 3 (out of perhaps 15 tested) actually made much improvement at all.
So my suggestion is to save the money you would have spent on additives, and use it to rebuild the engine when the time comes, if ever.
PS, in case it's not obvious, you'll know if the valve seats are receding because the valve lash will be too small every time you run the valves. On the engine that had problems with VSR, the exhaust valves would get so tight that it wouldn't idle, in just 5000 miles or so.