However, as the day unfolds, I’m very aware of whether I need ISO 1600 any longer, or whether I can drop this down to, say, ISO 800 or lower.
My ISO is very dependent on lighting conditions.  While I’d love to not need 1600, I also want to make sure I’m able to shoot at a fast shutter speed even in low light.  However, as the day unfolds, I’m very aware of whether I need ISO 1600 any longer, or whether I can drop this down to, say, ISO 800 or lower.  Similarly, if I move to this new faster shutter speed while the light is still dim, I may need to increase the ISO to make sure I can get 1/500 of a second a f/4 or f/5.6 (generally I’m at my smallest possible f/number, which can vary from lens to lens).