I have a Pentair Quad-DE filter which is basically a quad style cartridge filter that is designed to use DE. The filter unit can be run without DE (not recommended by Pentair) and then it acts like a cartridge filter. I typically tear down the filter once per season, wash out all the old DE and fully rinse the cartridges.

The reason why I mention all this is because the manual specifically mentions the case where the cartridges can get "limed up" (pool industry term). Essentially that means that calcium (and, to a lesser extent, magnesium) can scale out into the cartridge filter fiber material as insoluble carbonates. "Limed up" cartridge media looks crusty white/grey and feels crunchy when you manipulate the pleats with your fingers. The fix for a limed up cartridge is to do an acid soak in 1:5 muriatic acid/water (as per the Pentair manual, other filter manufacturers might have different recommendations).

So, the pool store guy is not way off but I don't think his prediction is quite right. If you do store extra cartridges and rotate them as you say, then you should just add an acid soak (if the cartridge can handle it) if the filter becomes "limed up".