Discipline in art is a fundamental struggle to understand oneself, as much as to understand what one is drawing.
The initial blocking in of the primary light / dark pattern establishes the overall plastic (giving form to) quality of the shape.
In terms of composition and harmony the light dark pattern is of utmost importance. The Japanese term for dark/light harmony is Notan.
The initial blocking in sets up the Notan structure. Drawings and paintings that fail, and by this I mean works that are unengaging, fail due to a lack of clarity of structure.
Keep your inital blocking-in to one simple and unified mass of light and dark.