Classical training is the best way to learn the control necessary to play fiddle well. Most folk teachers will not teach proper technique, since they've never learned it--at least, not all of it. They are also more likely to emphasize repertoire rather than technique, or so my experience indicates. The result is that you'd be more likely to develop bad habits early which you may never overcome.

Sure, to play fiddle, you need to augment what you learn through a classical approach, or rather, focus on different elements. But the more control you have over intonation, bowing, finger positions and ornamentation, the more easily you can adapt to whatever playing styles you wish to focus on. I don't believe there are "bad" habits you'd pick up from classical playing, particularly if you know early on you wish to play fiddle styles. Even if your teacher wants you to be able to stick faithfully to the notes, dynamics and exact phrasing, there's nothing to prevent you from also improvising, or picking up tunes by ear, or experimenting with alternative bowing patterns and so forth. Faithful reproduction ensures that you develop proper control over specific facets; it isn't a straight-jacket.

I've taken up fiddle relatively late in life, and I'll never become more than a passable fiddler. But it was a no-brainer for me to choose whether to start with a folk or classical teacher. I know I can pick up most of what I need to know about folk fiddling just by listening to recordings: there's virtually nothing in the folk styles which doesn't exist in close form in classical technique. Hardly surprising, considering how much of the classical repertoire has grown out of folk forms.

Not to mention that there's a wealth of fantastic stuff in the classical repertoire going way beyond the harmonic and melodic limitations inherent in most folk music.