Only three files require you to recompile the interface.fla/.swf: LibrarySWF, ILibrarySWF, IMegaZine.
LibrarySWF is exclusively for the interface.fla.
If you change any other file (including ILibrarySWF and IMegaZine) you need to recompile megazine.fla/.swf.
Exception to the rule: if you change the interaction between the rest of the code and a class loaded directly from the library (Library.getInstanceOf) you'll have to make sure the element in the library supports said change, and then recompile the interface.fla. I.e. if you try to access an additional textfield somewhere, you'll have to add that textfield and recompile.