What a fascinating way to think about it! I've never seen anyone draw such a parallel before.
I start off in the 'Waterfall' category, as all of my thumbnails are useless to anyone but myself. That said, my sketches are tidy and presentable, especially if I'm working with a new background and therefore have to sketch the whole thing out. If I'm chasing actionable critique (as in, I'll be changing things about that piece based on the critique) I'll seek it out in the sketch stage. After sketch, I'm definitely in the Agile category.
Lineart, flats and colours are all very neat.
I find, as I improve as an artist, that I'm moving more and more toward the 'Waterfall' category. For instance, the reason my sketches are neat is because I'm worried I'll mess things up at the lineart stage. As I become faster and more proficient, that fear goes away and I occasionally skip over definition in the sketch, preferring to add it during the lineart pass instead.