http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/6/html/Security_Guide/sect-Security_Guide-LUKS_Disk_Encryption-Manually_Encrypting_Directories-Step_by_Step_Instructions.html

Step 5

dd if=/dev/urandom ... is used rather than the equivalent shred,
which is much faster, and also gives a progress report.
Also the following note is less than informative,
and is a bit facetious in telling users to leave the command
running overnight!

I've updated step 5 at the fedora wiki as to how I think it should read:
https://fedoraproject.org/wiki/Implementing_LUKS_Disk_Encryption
(I wouldn't include the mention of scrub or wipe from there)

Note I'm quite sure the PRNG within shred is good enough for this operation:
http://burtleburtle.net/bob/rand/isaacafa.html