I'm sure I read that it is recommended somewhere whilst investigating bitcoin.
What you might have read is that if you have any old backup copies of your wallet.dat prior to protecting the wallet using the passphrase encryption that those old backups could still be used to spend the funds received to those addresses.
Even though you've now encrypted your wallet, the private keys used previously are still stored in the encrypted wallet -- they just are protected with encryption there now.
So to protect even against the risk of loss due to those old backups, what you can do is spend the funds in your wallet to a brand new address from the same wallet after you've added passphrase encryption. Then your old backups would not have any addresses that still have any funds on them.
