Oh....nylons and vaseline....ummmmm
Ok I'm back. If I feel a pic is going to be a candidate for diffusion I take 2 shots. One with a diffuser on the lens and the other without. That way when I am in the darkroom I can judge which effect works best. As Ole mentioned if you diffuse when printing darks blead into lights. When done in the camera its the other way around. So I get to choose which effect works best for the subject.