All you need to do is to keep your passphrase safe. That's the only backup you need. It's very important to save the passphrase in a secure place (or two).
Coin Wallet implements BIP39 which defines how your passphrase translates to your master private key. Using this master private key Coin Wallet derives child private keys for every coin you have in wallet.
Bitcoin family child private keys are derived using BIP32 specification, which specifies how the chain of child keys can derive in a deterministic manner from the master key. It means that you could use your passphrase to unlock your wallet with any wallet service that implements the same protocol. Theoretically, backups of individual child keys are not necessary.