Be VERY careful if you choose to do this... you need to be aware that (for Bitcoin Core HD wallets) as soon as you add (or change) a password to Bitcoin Core, it generates a completely new seed... so any backups from BEFORE the password is added (or changed) will likely not generate the same addresses as the new wallet.dat.
refer:
So, your backup of unencrypted wallet.dat is rendered obsolete if you encrypt (or change the password). Also, because the new seed is randomly generated, if you restore an old unencrypted backup and add exactly the same password again, it will generate a different seed and different addresses to the previous encrypted wallet.dat!
Note that even after the wallet.dat is encrypted for the first time (and on subsequent password changes), it "stores" all the previously generated private keys/addresses, so you won't lose anything... as long as everytime you add or change the password you make new backups.
Your best bet is to create multiple backups of your encrypted wallet.dat... store them on different media and/or in different locations etc.