How do you know when the primer pocket is stretched too much for the case to be used anymore? Are we only worried about the primer staying where it is seated until the shell is fired? or is it a safety concern?
6mm Shooter, If the new primer goes in easier than it did on original brass, then you've stretched your primer pocket and that brass should be discarded. If you are getting primer stretch, you are OVER pressure for that cartridge. Back off your load or change something to reduce pressure. Stretched primer pockets are not a sign of slight pressure, but rather big pressure overage.
They are stretched too much if the new primers fall out on their own. If they ever leak toss the brass. I have some really used up brass that still works fine but the primer pockets are very loose due to a bunch of abuse when they were new.