back when nikons had noisy ccd sensors controlling iso speed was very important. 400 iso was too noisy for some and 800 was too noisy for just about everyone. CMOS makes that concern a thing of the past.
I would use auto iso routinely and,maybe, set an upper limit of 1600. On my old d300 1600 would be the beginning of objectionable noise. with todays sensors, maybe you can do higher with no downside.