Backup the whole dir, from the github:
Byteball uses a single extended private key for all wallets, BIP44 is used for wallet address derivation. There is a BIP39 mnemonic for backing up the wallet key, but it is not enough. Private payments and co-signers of multisig wallets are stored only in the app's data directory, which you have to back up manually.
Should I wait until the transefer arrives or I can backup now and the transfer will be backed up anyway?