Well, i'm not the best person to advice you but i can tell you for me what makes sense.
First, if you are using AS3 port it will depends if is multicore or Standard. When i developed my first app i always try to make my code more reusable possible and separate it into modules.
In my first application what I did was as follows:
Where had to declare a notification it was always in place that made the code more reusable, that is, imagine you have a "module" Login where you need to declare the following Notifications: "TRY_LOGIN", "LOGIN_SUCCESS" and "LOGIN_FAILED" from my point of view i declare them in LoginProxy because it is something about the Login.
In case such an application to have a ViewStack which has two components such as Login and then UserManager, here i declared the notifications in ApplicationFacade because both modules did not "know" each other, but the application has to be aware of them.
I'm studying MultiCore but for me when i start with multicore make sense declare some Notification in ApplicationFacade and other in Proxy of each Module (for me i think it always depend on the kind of the notification to declare).
I do not know if you understand my "philosophy" that thought and that seemed the right one, probably can not even be the better way.
I hope I have helped, and don't forget that is just my point of view