If you are using a deterministic wallet which makes use of armory client, when you generate new accounts inside a wallet you will only have to make a back-up once. The backup key gives armory the possibility to create an indefinite series of addresses for your wallet. By default, Armory only generates 100 addresses, which implies that when you use more than 100 addresses and you lose your wallet, after you restore the back-up, the client will at first only see the first 100 addresses. You will have to specifically tell Armory to look for more addresses.

If you import private keys to a wallet, you will have to back-up them separately! The easy back-up process only works for addresses generated by Armory.