It will always work for the private keys that are included in the back-up. But remember, if you regularly create new addresses, make transactions, etc, you must back up your wallet regularly. Otherwise, new private keys generated may not be included in your original back-up.
Thanks, yes this is the point, how many transaction can be done before the old wallet.dat must be updated?
Deterministic wallets allow you to create unlimited amount of addresses with the same original backup.
+1.
It is very easy to make backups with deterministic wallets (like Armory and Electrum) as you only need to backup the seed.
Plus, the seed would be enough to generate all the addresses you created and to be created in the future.
Also adding to this, once you have a paper backup of your wallet with the seed, you can restore the wallet anytime even if you forget your passphrase on the wallet.dat file. So many people post here stating they forgot their password or must have entered it wrong.
