Well, so far so good for my kids.
Breastmilk is still acceptable cos no sugar ... lol
If it is formula milk, yes, you will have to make it a habit for them to brush their teeth after milk time at night to prevent tooth decay.
My #3 and #4 still latch and they are fine with beautiful teeths.
My #3 is 2 yrs old and #4 is 9 mths old.