The FM station where I DJ'd back in the 80s had a brandy-new 4-400A sitting on a shelf, internal structure clearly askew from being dropped or otherwise mishandled (maybe the Tube shaker stopped by?) with a big note on it that said something like "THIS IS WHAT HAPPENS WHEN UNTRAINED PEOPLE HANDLE TRANSMITTER TUBES". It disappeared shortly thereafter, probably by the culprit who got sick of being reminded.
As to 'rotating' spare tubes, that's basically nonsense and, as Jim referenced, just another opportunity for damage from mishandling. Last year I opened a pair of NIB 4-400s with 1989 date codes. Popped them in the transmitter, let them cook for a while, and (amazingly) - they worked just like new tubes!
Not bad for sitting unused 24 years or so.
When it comes to spares or really any tubes, there are two kinds: Good ones and bad ones. Okay, maybe three - bad ones waiting to happen. Perhaps all tubes eventually fall into that category if not used. Aside from surplus WWII tubes like the 250TH made with lower grade materials that contaminate the vacuum over time and/or become brittle, or an otherwise-specific bad batch, I can think of no situation where a tube that was properly gettered/evacutated/sealed went bad from sitting. Mishandling? Sure.
So rotate away, Flip.
