Hi Jon -
You should only need to do that on thin objects that you need to see both side of. For something like a poster on a wall for example, you're never going to see the back of it, so it's safe to keep it one sided. Any object that is already "solid" won't need it.
Just mess around and test out different situations a bit, and you'll figure out when it's useful and when it's wasting polygons.
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