I mostly use the Auto ISO feature, but I only allow it to increase until ISO 400, I don't like noise that much, and within this range the noise is pretty much nonexistant. For indoors action, in dim lights and so on I turn off AI and use 800/1600 ISO settings, especially when I can't use a flash. When I go manual (night photography, testing other stuff - like candle lights and so forth), I only use the minimum ISO my cam allows, which is 100 ... so as I can see its pretty much situational.
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