Are all View Models supposed to be accessed through the Main View Model in MVVM?

Question

I am currently working on a WP8 application. My current design is to have each view bind against a specific view model directly.

Looking through the samples though, it seems that another way is to have all the view models accessed through the Main View Model and then have all the views to their view models through the MVM.

Is this the correct way to do it (So that it doesn't cause flexibility and other issues in the future)?