I have to admit I do not detail strip the 870's every time I fire them, if its only a few rounds a quick clean is all it gets. The joy of revolvers and pump guns.
Once every 6 months or so they are cleaned 100% including removal of the fire control group and cleaning of the mag tube. Mag spring is on once every year and a half or so replacement.. usually be then it is looking ratty enough I wonder about it though I have never noted an issue.
My guns are parkerized and so they hold oil on the surface a bit, if I have not been out shooting them the amount of house dust and dog fur gives a good clue that it's time for a ready and re-inspect.
Usually once ammo is cycled through once or twice it goes in the practice bin and new is loaded. Likely a non-issue with modern shells but why risk it. I use the cheapest slugs from major US makers. Though I have an interest in that is better or best when it comes to handgun ammo, there is not a study out there that can convince me that it matters at all what slug one uses at defensive ranges.
Accuracy and penetration when it comes to hunting would be a whole other story.