I went through the same valve-seat paranoia with my /5. Unleaded gasoline was phased out in the late 1970's and the pre-Internet discussions in BMW Club newsletters was how unleaded fuel was causing severe valve recession. So around 1983 I went and did a "unleaded valve seat" replacement. Turned out that my /5 was fine, it was changes in the valves that caused the problems in the "new" early 1980 BMWs.
I'll admit that I try to use ethanol-free gas when I can, simply because I had an old trank liner (Kreem) that is not alcohol-proof. I plan to remove and replace that tank liner when I get my tank painted next year.
Keep an eye on things, you should be good. And if something goes amiss, repair it.
'73 R60/5 Toaster
Luddite. Not just a philosophy, a way of life...