In reality, you don't even need to keep a wallet.dat file if you just need a cold storage Bitcoin address. Storing your cold storage address on a clean computer that has never been connected to the Internet is indeed a secure solution, but it's much less secure than not having a digital copy of your wallet at all. Since you won't be touching the funds on that address anyway (at least not for the foreseeable future), there is no reason to make it accessible to any Bitcoin client. Personally, what I would do is extract the private key of a newly generated address and write it down on a piece of paper. I would then store that piece of paper somewhere safe. But for most intents and purposes, assuming that you are not a high value target, then yes, your solution is secure enough.