You should definitely shred files you are no longer using, even if they are encrypted. If you use an encrypted volume, the non-shredded files can still be recovered even after they are deleted, if the volume is mounted when you are attacked. Shredding files removes them entirely, and they can no longer be recovered even if the attacker gets you with the encrypted volume mounted. In the case of singly encrypted files, shredding them removes your ability to decrypt them if you are court ordered to do so, whereas if they are not shredded you could face being held in contempt of court for not decrypting them.