I don't know if my google-fu is broken today or something, but I can't seem to find any information about why I would or wouldn't want to erase and format an internal boot drive and/or partition thereof as Journaled, Extended prior to doing a clean OS install. Everything I've come across only mentions using Disk Utility to format an external storage drive as Journaled, Encrypted, using Disk Utility to format a boot drive/partition as Journaled or using File Vault to encrypt an already formatted drive. What differences are there between formatting the drive and partition as Journaled, Encrypted and then, also, turning on File Vault vs. formatting the drive and partition as simply Journaled and then turning on File Vault?
If you format a drive as Encrypted, you are in fact turning on File Vault, as that's the only encryption layer that OS X supports. The main advantage of partitioning the drive as Journaled, and then turning on File Vault is that you can revert the drive to a simple unencrypted volume if you should choose to do so in the future. You can't do this with a volume that was formatted directly as Encrypted. Other than that, the end result is the same.