You can use “auto ISO” if you like – but never in a studio or when using manual lights.
Otherwise, if you like, you can use it, since other than in terms of grain, ISO does not affect the essence of your image. (That to me is the measure of what you should do manually: anything that affects the look of your image.)
When you use manual ISO, here’s some simple starting points that will help you get off on the right foot:
- Outdoors, or when using tripod: 200 ISO
- Indoors, even when using flash: 400
- “Difficult light”, or sports: 800
- But.. go as high as you need to get acceptable shutter speed!
Those starting points are just that – but they will start you off not far from where you need to be.
ISO: raise it when your pictures get motion blurred. Lower it when you need great quality and have light or a tripod.