Cheating is wrong, always.
While this may be technically correct, I think we can agree there is a pretty wide spectrum of wrongness.
If you find a dollar and don't declare it as miscellaneous income your are technically a tax cheat even though no one knows, or cares. Its 'OK'.
If you've been married 10 years and the old lady has put on an average of one pound a month since the wedding day, sneaking some strange in your weight class is a lot different than banging the Maid of Honor on your wedding night.