Hi E.!
Ahhhh...how sweet! Even when they begin to get "big", they still show how pure they are don't they?
My opinion is pretty simple. I believe if she needs braces anyway, then DON'T let this be why you take this security away from her. HOWEVER, she's getting to the age where she will start Kindergarten soon, and your concern should be if she were to do it in class. Most kids "shy away" from doing things like this during school time, but you can help her early on.
I would suggest something simple. Whenever you see her starting to suck her fingers, then sweetly tell her "oh honey, you must be tired after playing outside, why don't you go lay in your room for a couple of minutes"..
If you begin to keep the "sucking" in her room only, then she will begin to NOT do it outside of her room. Especially because it's a "sleepy thing" anyway.
We did this with our son's pacifier....a firm addiction! Anyway, it worked. I never let him think he was wrong by wanting it. I always said, "sure you can have your pacifier, just go in your room with it, because it means you're tired ,blah, blah, blah"....He eventually got too "busy" to take the time to go to his room for it. It tapered off on it's own....thank goodness :o)
Anyway, I hope this helps you in some way.
:o) N.