In my opinion, and this is what I tell my students in my FA/CCW classes, you shoot off the defensive ammo you carry with you, once a year. The best way to set this practice up is in the spring, when Daylight Saving Time kicks in and you need to reset your clocks and check the batteries in your smoke detectors.
The reason is as you suspect: the ammo you carry in your EDC can get contaminated with lubricants, sweat, and other contaminants, and when your life depends on it, there is a possibility -albeit very small but there nonetheless- that the ammo in the gun may not perform as requried. I have even seen things like lint filling the fluid cavity in hollow-points (from unloading the SD ammo at the range, putting it in a pocket, shooting with FMJs, then reloading the HP back into the mags after range time).
Properly stored ammunition can last like 20+ years....keep away from humidity and extremes in temperature and they should be fine. Regularly inspect them, though, and when the ammo starts to smell like ammonia (usually an indication that the gunpowder is chemically deteriorating), it is time to shoot it ALL off, otherwise you are good for continued storage.
Remember, springtime = daylight saving time, check smoke detectors, blow off the ammo you carry daily. No need to blow it all off, just what is on you (what you carry in the gun, in the car, in spare magazines).
My opinion. YMMV.
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