As far as I understand it, for the vast majority of non-metals you can leave reflection at 1 and IOR at 1.52.
For metals it's a bit different - the way I work with metals is Diffuse Level set to 0, Siger Shaders Complex Fresnel in the Reflection Colour Slot as a starting point, Corona Color in the IOR slot set to 99999, 99999, 99999 for RGB - which effectively disables the IOR, at this point I'll fire up Interactive Render and tweak the colour of the metal using the settings available in Complex Fresnel and then the blurriness of reflections using the Reflection Gloss value until it looks right to me and/or my reference.
Of course this is only for 'pure' untouched materials. If I want to add a bit of realism then I also add some Gloss Maps into the mix and or Layered Materials.
Whether I'm doing things right or wrong, I don't particularly care, as long as it looks good/correct to me and the people paying for it.