It is advisable to use nuts with a nylon insert to avoid self-loosening (although I doubt that this will happen).
You need a lot of vibrations for self-loosening a nut. From the tests I've seen, nylon inserts help a bit but given enough vibrations, it'll still get loose. None of this should be a problem for your seed word storage though, unless maybe if you store it on top of your car engine for years.