My first SKS firing pin was seized in the bolt assembly so badly that even when I hammered out the retaining pin, the firing pin would not move even a fraction of a millimeter until I took a set of pliers and forcefully pulled it out. It was caked in all sorts of debris - not just cosmo, but black flecks of some crud granules, plus some coffee sludge (cosmo and other grime). After spending quite some time cleaning the pin and especially the channel, I left it super dry. Have fired over 900 rounds since with no issues, and always take the bolt out and shake it to hear the rattle. Recently decided to take it apart again to check, and despite using pretty dirty Norinco ammo, which made everything else dirty - the firing pin surprisingly was not too bad and there was nothing caked onto it. I inspected it for any wear or damage. Nothing. So I am going to keep it running dry.
Since then I have purchased a few more SKSes and one of them I left in cosmo except for the bolt assembly which I took apart and while the firing pin wasn't seized as badly, it still would not free-float move with shaking - so just cleaned it just for the sake of it and left it dry even though I had no intention of using the rifle due to its very nice condition.