My 7 yr old daughter sucked her thumb until age 4.5. I agree with the mom who mentioned the teeth moving back...especially if your dentist is not worried...I would not worry about her teeth being affected yet.
My parents nagged me about some things when I was little, and I refused to go crazy on this issue. We started talking about stopping when she was about 4 yrs 3 months, gently and she herself decided she'd stop. The nightime was hard, our dentist had us use medical adhesive tape around a popsicle stick broken so it went over her thumb on both sides to help keep it out of her mouth when she slept (we did this about age 4 yrs 9 months). She was so good, she'd even remind us to put the tape on her thumb if we put her to bed and forgot and even would remind us at night. Her thumbsucking went away so fast...I can hardly remember now, and it is a sweet memory of a cute little baby habit.
So our experience is that you should let them self soothe still for another year or so and then start talking about being a big girl and trying to stop, and how can you help her, should you remind her, etc.
don't make it a big nagging battle.
Lots of love to your sweet baby girl!
Andre