Effectively, it will have no effect, if no light passes through the material at all.  but translucent (ie. partially transparent) materials will use it.
If you look at the Material Editor's settings for various 'solid' materials, you'll see there is no effect whether the RI is at 0 or 300.....So save yourself some calculation time and give solid materials a zero setting.

RosemaryR
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"This...this is magnificent!"
"Oh, yeah. Ooooo. Aaaaah. That's how it starts.
Then, later, there's ...running. And....screaming."