If you generate new addresses, new private keys for those addresses are created and hence you will need to backup your wallet.dat file everytime you do that and a transaction happens on one of those addresses to safeguard against missing funds. You can and should automate backups of the .dat file by using a cron script or if on windows create a scheduled task to backup to a secure location which only you can access to and no network access is available.
The idea with the backup here is to ensure that if a transaction happens on the addresses you generate (may it be the segwit address or legacy) you should make your backups to ensure that you have the latest wallet.dat file which contains the private keys to it.
This was the case with the old format, as far as I know, since bip32 was introduced (I think this is the bip for the current HD format) then you no longer need to be making backups each time, but I don't know how this would be the case to be honest.
Even if HD mode is enabled, and you make receive a transaction with a newly generated key, how can a former HD enabled wallet.dat know that this transaction was received, if the key was never generated? same goes for sent coins. This confuses me a great deal. I think it has something to do with pre-generated addresses that you don't see but already are there or something along the lines... it's weird to think for a non coder. I would like achow101 to explain this in laymans.
My advice is that even if you are using HD mode, you keep making backups and don't delete previous backups, just rename them.