L.B.
YES! break that bad habit now! My Dr. told me not to worry about my daughters thumb sucking, he said infants and toddlers use thiumb sucking as a comfort item and most grow out of the need befor it does any permanant damage. Well i now have a 10 year old that needs jaw realignment, braces, and possable wiring and she STILL sucks her thumb in her sleep! It has become a bad habbit that she now does unconsiously not because she wants to (you need to understand that she is very embarressed and does NOT want to be a closet thumb sucker) I wish i would have gone with my gut (and mothers advise) and not listened to my DR about this issue.
Good luck