That is somewhat debatable. I've taught simple effects to different audience members because I believed they had and interest. I would teach them a few card sleights, and see where they would go from there. Nothing major, but enough to quench their taste for the time being. Every once in awhile, I would run into one of these people, and they would show me the effect that I taught them, and sometimes, they would show me what they had learned since. So I figure it can't hurt.
Now, I personally suggest that you teach them simple card tricks. Nothing to big, but just enough, such as a simple ACR. Then wait awhile and see what they do with it. If they improve and stick to it, then feel free to give them just a little more, or guide them to sources that they may be interested in. If they loose interest, then no harm done.
I have always felt a bit greedy when I don't teach some sort of simple effect when spectators ask and burl into me to teach them something. I do this because I am following one of the rules of magic: never reveal the secret to an effect. I am curious how other magicians of the forum feel about this? This could be one of those grey area sort of things.