You know, I cringe when I see it. I just wrote a check the other day that was numbered 666. I know it is supposed to be bad and mark of Satan, but not sure if it's even in the Bible.... It's one of those things I grew up knowing that it's bad, but not why it is.
Revelation 13:18 (Whole Chapter)
[Revelation 17:9 ] This calls for wisdom: let the one who has understanding calculate the number of the beast, for it is the number [Revelation 21:17] of a man, and his number is 666. [Some manuscripts 616]