I think alot of these type of things get started by the uniformed and/or uneducated, who come to these conclusions in an illogical way.
I was "taught" something very similar regarding knives. "Folk" logic in the area in which I work says to rub onion or garlic on the knife to make the wound unable to heal. When I asked how they know this, the answer is along the lines of "my uncle/grandfather said it, and he cut a guy in a fight one time".....
Its a very UN-scientific process that they come to these conclusions. If a person bleeds alot after you rubbed oil or onion on your blade, does that mean that the oil caused that to happen? Too many variables....