Everything-blog “Mental Floss” thought it'd be interesting to investigate this issue, and called upon two “experts” in the Nintendo cartridge-blowing field to confirm or deny said heresy. The first, a man who produced “Ecstasy of Order: The Tetris Masters,” a documenary about crazy nintendo Tetris legends, noted the following.
“[Blowing in the cartridge] is actually terrible for the games and makes the contacts rust. You’re really not supposed to do it. But it works. [laughs]” This sums up the problem: although intellectually we knew that blowing into electronics was bad, we did it anyway. It seemed to work.
Of course, this is more speculation than fact. Fully committed to crushing our dreams however, Mental Floss called upon Frankie Viturello, who actually conducted a study on the issue. Virtuello stripped down two copies of Gyromite, kept them in the same place, and blew on one 10x per day for one month, and left the other one alone. You could read the results here, but the pictures demonstrate that the blown on cartridge is significantly more damanged than the other one.
Finally, we have Nintendo itself, who apparently needed to lay down the death hammer:
I feel like this is very similar to a friend telling you that your girl is hooking up with another guy, and simply refusing to beleive it. Despite what seems to be real evidence, wouldn't you just know if things were no longer working out? Crazy.