Well, depends how nuts you wanna get about it. Just reflashing will work, and I wouldn't bother with more; although a forensics expert might be able to get stuff out, it's a major undertaking as the medium is not readily accessible.

If you want a little protection, write a couple random files of the right size for the root fs, and flash it with them successively, then flash the filesystem on. But to me that's putting write cycles (if not very many) on the flash for no significant gain.