This is an interesting article and unscientific study into the effectiveness of blowing into the cartridge. It shows whether or not our "this will fix it" trick actually did anything. I won't spoil the answer:
I remember using this trick all of the time for my first and love-of-my-life system, my N64. I think we might be the last generation of gamers to know this trick as all the new game systems are digital or disk. It's kind of sad knowing a whole generation of gamers won't know the joys of the classic games and/or game consoles.
And in case anyone is wondering, my roommate Zach still blows.