With FileVault enabled, do I need to overwrite my hard drive with 0's before getting rid of it?
No - the point of FileVault 2 is to make it extremely difficult for a motivated person to decipher what was stored on that volume.

I would pay more attention to destroying that key that was used encrypt the data than the drive.

Of course, you could zero the drive, but why not go one better and pick a new encryption key that was never used, and never will be used and let your machine re-do the encryption of the drive once you have erased all the data one pass.

Since there have been no scholarly articles on how easy it is to decrypt a FileVault drive nor reports of exploits - you have to weigh the potential damage that leak of information could be. FileVault makes it much easier to not worry if you lose a drive, so the old saw to be sure to overwrite things many times is far less applicable for drives where encryption exists at rest.