There are no rules. However, we teach a process that focuses on developing the loud conversation first. Because the loud conversation is made up of the shapes or areas that are the highest contrast, this conversation (not the quiet conversation) will be the most noticeable and therefore, affect the art more than the quiet conversation.
Keeping our focus on the loud conversation first helps us to make better decisions regarding our composition, and to see, early on, what our final design may look like.
If we fully resolve the quiet conversation first, then begin to add a stronger set of contrasting values (the loud conversation), our whole composition would change as we would be moving these bolder, more contrasting shapes around until our loud conversation was resolved. This construction process will probably end up ruining and covering up all that refined, subtle work we had done on the quiet conversation in the beginning of our process. It is quicker and more helpful to determine the basic loud conversation in your art first. If the loud conversation is weak, no amount of fussing with subtlety in the quiet conversation will change the composition from weak to strong. This can only be done with the loud conversation.