Use it at your own risk. The only containers that Bethesda has gone on record as not resetting their contents ever are containers in a house you have bought from one of the Stewards of the 5 keeps in Skyrim.
If you are going to use any other container I recommend you at least test it once by saving a game first, putting some items in it, and going to another city where you wait for 2 weeks before returning to the container to see if your items are still in there. If they are, you can reload the game and play from there, but I'm not saying that means the container is safe as it could simply have a longer time cycle for resetting. In general I would drop by and check the container every few days or so if possible because checking the container will restart any countdown timer for the container.
