is it safe to store things there?
No containers that you can put something into should respawn, the respawn script disables adding things. Now, sometimes the items in the container will be random from a list, but it is set once you open/activate it. So, if you were to load a cell again, you might get different stuff. No, it won't be any better, maybe you get two blood bags instead of one stimpack that kind of thing.
However until we load up the geck and take a look see, I would hazard a guess that anywhere the game tells you to make yourself at home and you see blue is all yours for the pack ratting. However I would like to verify that. For sure, I would avoid any containers that indicate ownership for storage, it might transfer ownership and then you take a Karma hit for getting your stuff back.
There are also those shipping boxes, not sure how that will work in game as far as integration to how I play.