For HD wallets, the seed is usually all that you need to back up. The seed will generate each and every of the addresses that you could use within that specific wallet. This applies for wallets such as Electrum.
For Bitcoin Core with hierarchical deterministic enabled, they don't utilise the mnemoric and hence, you have to back up the wallet.dat at the start. If you're using Bitcoin Core without it, you will have to backup every 1000 transactions, if you are using the latest version. *
* You do have to backups Bitcoin Core every single time the wallet gets : encrypted, or the password gets changed.