Hickory: If you are playing vanilla, there is absolutely no point in dual classing her. If you are playing TuTu, there is absolutely no point in dual classing her. If you are playing BGT, you *could* dual class her for the sake of continuity, but there is still a strong reason not to: she will lose her thieving abilities for too long a time, and crucially, in your case, she will have to attain a full six levels before she gets her thieving skills back. Even if you are playing BGT, I say don't. You can always dual class her once you reach BG2, where she will be able to catch up MUCH faster, since XP is thrown at you at an amazing rate in BG2. So bottom line: don't.
advancedhero: Okay, thanks. I am playing Vanilla. I don't like playing fan-mods, at least until I beat the game once in it's original form.
Well, if you plan to continue to BG2, then the characters (other than your own) will be given new stats and xp levels according to where the designers felt most players would have reached, rather than reading them from your save. The version of Imoen in BG2 dual classed (into mage, obviously) at around level 7 or so. If you want to retain continuity you could dual class her then, or a little earlier. On the other hand, your BG1 game will be significantly easier if you don't dual class her, and just imagine she learned magic abilities in the interim between the games. If you're only playing BG1, there's little point in dual classing her, as Hickory says. Also, if you're only playing BG1, you should definitely try BG2 once you're done :P