I'm concerned about my cathodes sputtering away after several years. My big clock has tubes with a single-digit illuminated 24/7, which is the worst-case scenario. The Burroughs datasheet documents testing done circa 1960, and they observed basically no failures on several batches of tubes after 30,000 hours (almost 4 years). By rotating the tubes, I would expect I could maximize their lifetime by evening-out the wear on the cathodes, assuming that sputtering is the death-mechanism. If I were to wail until a tube failed, presumably from sputtering, there is a higher risk that the unused cathodes are poisoned (from their lack of usage).
So I see it as a tradeoff between (a) mechanical damage from resocketing, (b) cathode wearout. I just dont know how to optimize between these 2.
6091 nixies are getting hard to find, not to mention pricey. My clock has 15 of them, and I dont want to dip into the few spares I have.