I don’t know if anybody knows with certainty how accurate these boundaries are. Cree dialect studies still need — and desserve — a lot of work. This map is useful, though, in creating a rough picture of the Cree language continuum that runs from the Rockies to Labrador; as far south as Montana, and north to Hudson Bay.
nêhiyawêwin is the Cree name for the y-dialect, called Plains Cree (olive green) on this map.
Yes, there are differences among the dialects, but there is much to be gained if speakers and teachers can find ways to cooperate and share resources.
Here is a link to the Wikipedia article from which the map is extracted. it also discusses the delimiters of dialect (or language) as we currently (think we) understand them. http://en.wikipedia.org/wiki/Cree_language