Don't let localities or individuals decide what way they will be medicated. Just make sweeping decrees for everybody even though it is not in the traditional realm of “public health” as in communicable diseases.
Libertarian Solution: Fluoridated salt and non-fluoridated salt
I have traveled in third and second world countries. In many places people can buy either the non-fluoridated salt or the fluoridated. Each parent decides for their own family. The cost for each of the types of salt is the same.