you should only really dither when trying to give the impression of a rough texture, especially when your final image is doubled.
just add an extra colour if you want it to look smoother
I'd say this is bullshit. You can perfectly use dithering for blending colors, giving volume, smoothing and so on, and not for just giving "the impression of rough textures". It pretty much involves mixing dithering patterns (not only sticking to one, though you can if desired), knowing light sources and the goal of dithering itself within the picture, and of course knowing how to do it :X