I'm not so worried about o-rings, food grade o-rings are easy to find and replace, you just need to know the measurements. But for just the convenience factor it's not a bad idea to pick up a few backups.
I was referring to subtank mini seals, which are specifically shaped gaskets rather than off the shelf o rings.
This means that if you are stocking up against the possibility of parts becoming unavailable, you would need to buy them in advance and store them for long periods.
This is why I recommend against the subtank mini for stockpiling.
Most tanks take regular o rings, which will still be available, so long term storage is not an issue.