Perhaps your nixie display driver needs to keep statistics on each tube and each digit, and when it goes into "screensaver" mode it exercises the unused digits over the others?
not too hard to do, actually. as long as the clock controller has enough ram and code space to support that. sometimes I fill up the flash with app code and there is not much space left for extras like that.
if you are going to use it as ONLY a clock, you probably don't need to care. having bad digits that never turn on anyway is not going to affect the good digits. the russian in16's that I use have been on for about 2 yrs constantly, now, and I see no signs of any problems on the 'clock digits'. I have not tried the non-clock digits on THAT clock (the 6digit one I posted) but on the 4 digit one (using US tubes, not russian) those get poisoned very quickly and I did use that clock as a generic number display device and even after 2 weeks of 'clock use' I saw badness in the non-clock digits. I would probably have a nightly routine that leaves the bad digits on for maybe half an hour at a time and cycle between only the baddies; the goodies get enough time during the day to be fine
if your tubes are in a socket, its not so bad. on my builds, they are all hand wired perf and so replacing a tube is NOT something I look forward to. if/when I go to a pcb, it won't be a problem.
I've also noticed that the direction you go, matters. I had a semi-bad 7, iirc, and the top flat part of the 7 would come on if I went from 8 to 7 instead of 6 to 7 (I may have the specifics wrong, though). I noticed this when I had a volume control (preamp system, DIY) send vol change messages to the clock and if I turned the knob up or down, I could see some of the dim segments come on or NOT come on, depending on what the previous lit thing was. it is not really related to the numerical higher or lower, but the one that is next to the digit, in terms of metal and glass and gas
but I did see the thing act differently if hit from above vs below, so to speak. so the lesson in that is, how you do the exercising matters as much as the duration.
if you find actual data on this, I'd love to see it; but I have not found a definitive guide on how to prevent CP on the nixies. no one wants to say exactly what their exercise routine consists of and what they tried and found didn't work.