Is it necessary to wipe (using shred or similar) the drive or partition again even though it was already encrypted? Threat model for securing the drive is for potentially physically losing the drive and files getting stolen.
Modern encryption schemes are trying to achieve ciphertext indistinguishability (AES and others). It means that you cannot distinguish a ciphertext from a random sequence of bytes (no more than with probability of 50%).
This said, if your disk was encrypted, and if you consider that your opponents cannot break the cipher algorithm that you were using (at least not before the data in your disk will becomes obsolete), you can leave your disk as it is.
But, you definitely need to erase (and shred), all the key information (key slots) from any places where it might have been stored (because a physical access may lead to the recovery of it if a flaw is discovered).
Whatever, my personal advice is:
Shredding does not take so much work (start it on a Friday afternoon and get your disks back on Monday morning clean and ready to be used). Just do not try to optimize security, it is just wrong.