Another person had established the policy that before a disk can be wiped it must be decrypted first to protect the MBR.
In addition to Adnan's answer, I'm not sure where this policy comes from. The MBR (or the GPT) is not an area of the disk that if somehow damaged cannot be repaired. What damaging the MBR, or GPT, would do is render any defined partitions on the disk unidentifiable. The MBR or GPT after all contains the partition table, so without it you're pretty stuffed for identifying where the partitions start, especially if they're encrypted.
Note that with the GPT, there's a backup partition table at the back of the disk, so you'll want to nuke that too.
If you accidentally (or deliberately) obliterate the MBR or GPT, many partitioning tools can recreate one - of course, only specialised tools can detect partitions on a disk without a working partition table (I think testdisk can guess at the partition layout, for example). In GParted, you can select "Device" then "Create partition table" to make one. Windows will also do this as part of its install on a disk containing no such table.
however the policy got changed and now we simply enter a command to fix the MBR (something like MS-FIXMB). Can this process be optimized any further or can it be shown that this step isn't necessary?
The good news is that the MBR lives outside the encrypted disk, so you won't "damage" the MBR by not decrypting the disk (although since you want to wipe the disk, who cares - take the partition table out too!). Most software whole disk encryption solutions work by writing their own code to the MBR and likely to the no-mans land between the MBR and your first partiton (there's quite a bit of space in there, believe it or not) for the purposes of decrypting the partition initially to get the next stages (volume boot record, bootmgr) kicked off.
It is possible the partition table is also encrypted in such a scheme (the BIOS doesn't care about it, only operating systems do) meaning it would appear as one whole, empty disk when encrypted. The purpose of such a scheme would be to make it very hard to identify how the disk is partitioned without decrypting the disk, which re-iterates my point: destroy the partition table too!
Long story short:
- Overwriting the MBR won't make the disk unusable physically and is largely irrelevant if your intention is to kill the existing OS on the disk.
- You can easily reinitialise the disk and get an MBR back.
- Actually, destroying the partition table makes the job of anyone reading the raw disk a lot harder.
- The important aspect, which Adnan has dealt with, is to destroy the key material.
Disclaimer: I work producing a disk imaging product.