darkest to lightest or middle tone and branching out towards highlights and shadows are both good methods. Lightest to darkest however, is not preferred.
That makes a lot more sense now that I think of it. Our works are a lot like paintings. I use to draw the outline because I wanted to color something in but painting it just makes it look more beautiful.
A lot of us do subscribe to the "painting" method, it's an easy way to think in terms of 3-dimensional forms and can indeed make for some beautiful work. Line art can still be a useful technique, but it is probably of least use in organic background art/ tiles.
Even when you do have line art you shouldn't be thinking of " coloring things in" but thinking about how you can make something appear to be 3-dimensional.
