Or fill it with some AAA Steam games shouldnt take long to fill the whole thing.
cat /dev/urandom > /path/to/mount/point/temp.bin && fysnc && rm /path/to/mount/point/temp.bin
is much more reliable and isn't dependent on your internet. dd if=/dev/urandom of=/dev/sdX bs=1MWiping the encryption key takes care of that problem, as well as being much faster.
Sometimes I do just that, I'll install Linux encrypted, and then reinstall later without migrating any bytes. My main concern is work related / personal finance documents being left over on a laptop. I've tested plenty of forensic utilities on my system after erasing files.