The brush tool works wonders for applying shades under smaller, harder strokes as well. Fills it in as opposed to leaving blank white spaces untouched by the pen/pencil tools. I typically don't use it that way myself, but if I'm doing something really sketchy, it really eases the eye; I ususally just use the brush tool for shading, but I see you've got more of a traditional hard-shading technique as opposed to cell-shading.
