I may have gone down as far as 1% on fill lights but it all depends on your scene.  Are you looking for realism, realistic behavior, or a fantasy look?  For most lights, I turn on Inverse Square to simulate the actual falloff of real-life lighting, this is with the exception of Infinity lights, that feature is unavailable.  Since I render in Superfly, I convert all my lighting to Blackbody, I find them a little more realistic in my scenes.