The original question was really asking where to write to, that is,

rwd0c vs. wd0c; the source that was used in the example

(urandom/arandom) wasn't any kind of "true random" entropy anyway,

AFAIK, they are non-blocking pseudo-random stuff that the kernel

spills out...
I mean, as far as usability goes, it is just a matter of typing

if=/dev/urandom vs. if=/dev/zero, virtually no extra work needs to be

done by the human... and as far as the computational difference, I

think the delay for using pseudo random source is negligible when

people are probably have to leave this thing running overnight anyway.
So I don't see any big fuss about which source to use here, surely no

one is asking what's the best entropy to be used, but just how to

actually write to every bit of the hard drive.