I, for my situation, just didn't want a dad buying the hdds for his kid and an unnamed kid, that is a pc wiz, suddenly has a bag full of adult candy sprouting tentacles and whatever on their "new" disc. 
No need for 7 pass here then. Just a simple zero fill and that's it.
The 7 pass advice comes from the time of MFM hard drives. Those drives had quite big mechanical tolerances so the heads would not be exactly on the track when reading/writing, so a single overwrite would not overwrite the whole track. Also, the signal/noise ratio was quite large, a simple peak detector could reliably decode the data. You could then read the raw signal (before it is converted to 1 and 0), decode it, then encode it back (creating an "ideal" signal) and subtract the ideal signal from what you read hopefully getting something more than just noise. In theory it was possible to do so 7 times, though I doubt that all hard drives could do that, probably just the severely misaligned 5MB drives (lower capacity - bigger tolerances).
Modern drives operate at around 2dB signal/noise ratio ( (E)PRML is used to decode the data, no longer simple peak detection) and have tolerances that are really tight. It would not be possible to reconstruct even one overwritten layer.
