I've been looking at lots of the art here recently and wanted to point this out since we're a predominantly comic style-focused community, and shading might be undervalued a lot - pun intended! Seriously, you all should do it.
Quote from da Vinci: "Many are they who have a taste and love for drawing, but no talent; and this will be discernible in boys who are not diligent and never finish their drawings with shading."
Nothing's a better test of your depth perception, reasoning skills and muscular-visual coordination than a good shading exercise. If you try to do this and it doesn't come out looking right, you should work on it. Heck, I need lots of work on it.