If you use disk encryption in Linux, there's a good chance you're using LUKS, a standard implementation of it.
The first few bytes of your encrypted partition contain a LUKS header, which contains things like the cipher, cipher mode, passphrase iteration count, and salt.
Mine (standard CentOS 6) looks somewhat like this:
LUKS header information for /dev/sda2
Cipher name: aes
Cipher mode: xts-plain64
Hash spec: sha1
Payload offset: 4096
MK bits: 512
MK digest: 59 67 99 11 f7 1d d2 3d 66 90 60 15 08 27 8a 8d 2c 1b 4b 46
MK salt: c6 ff 58 17 9a f3 68 b8 c1 3c 81 af df ce 5e 65
3f 64 c9 70 fe 90 df df 25 ce 16 b9 f6 46 45 b8
MK iterations: 46875
Key Slot 0: ENABLED
Salt: 9d c6 a8 af 33 a6 d3 dd 1d 9c 53 bf 7a 4c 32 39
88 d0 93 da 88 5a 7b 51 0d 36 7e f0 08 7c 34 69
Key material offset: 8
AF stripes: 4000
Key Slot 1: DISABLED
Key Slot 2: DISABLED
Key Slot 3: DISABLED
Key Slot 4: DISABLED
Key Slot 5: DISABLED
Key Slot 6: DISABLED
Key Slot 7: DISABLED
These are all required (along with your passphrase/key device) to be able to decrypt your device. However, unlike the EXT superblock, there aren't any alternative copies of this header.
If it becomes corrupted in any way (a single bad block on your harddrive), knowing your passphrase won't help, and all the computing power in the world won't be able to recover your data.
Hence, it's probably a good idea to back it up.
cryptsetup luksHeaderBackup /dev/sda2 --header-backup-file=/boot/luks-header
I put mine on /boot, as this is an unencrypted partition, and the file is small (2MiB).
There's no great security loss in this - anyone with physical access (or root access) to your device can simply dump the header anyway. If you're really worried though, save it somewhere safe, or print it out, and store it somewhere.
While you're in the backing-up frame of mind, backup /etc/lvm. If your PVs, VGs, or LVs get messed up, you have a chance of being able to recover from this. I also back this up to /boot.
cp /etc/lvm /boot/lvm-backup -Rafv
This too is a small amount of data.
I suffer, so you won't have to.