Reading this confirms that relatively easy recovery of overwritten data on hard-drives is a myth. I would simply overwrite the data once and leave it at that.

The techniques described on that page are only used for military and government applications where data recovered - even the smallest amount - could reveal threats to national security. It's not for revealing that someone wiped the company accounts for malicious purposes, or that you've got infringing copies of copyright protected material.

But if it makes you sleep better because your drive has been wiped to some DoD standard that's fine. Bear in mind though, most of the companies selling this type of wiping software prey on the conscience of those who believe the hype that such recovery is possible.