The graininess doesn't change just because the shot gets darker -- if you had a scene in a well-lit bedroom and at the end, someone turns off most of the lights and the room goes very dim and dark, do you open up the lens just as the lights are going off so that the room stays fully exposed on the negative and then darken the shot again in post just as the room lights are going off?
There is a base graininess based on the exposure / ASA rating and how bright it is printed or transferred, and that base look stays the same as shots get brighter and darker within the scene more or less (it's true that the darker areas have fewer small grains between the big grains but it's hard to see that when that area stays dark.
It's when you lift the image to be brighter that the graininess will be increased.
Often a cinematographer will rate a stock slightly slower and print "down" this extra increase in density on the negative, or at minimum, a little bit of extra exposure helps compensate for when your exposure is a little off. Rating a stock slower doesn't make it less grainy technically -- the large and small grains are part of the design of the stock that gives it its speed. But overexposing allows more of the small, slow grains in between the large, fast grains to get exposed as well, filling in the gaps and giving you a tighter grain structure.
So for cinematographers trying to reduce graininess in Super-16, the general rules are to use a slower speed stock if possible, and also if possible, rate the stocks, or at least the faster ones, a little bit slower.
From that base, you would light and expose creatively for the look you want.
However, it is true that when talking about very dark scenes, most cinematographers who have experience with printing film, tend to be conservative about exposing to give themselves some flexibility in post. So maybe you feel that a 2-stop underexposure would be right to create the feeling of dimness you want, but on the day, you underexpose by 1 1/2-stops and tell yourself that you will finish the darkening in timing. Or maybe for this dark scene, you rate the stock a little slower as a base but then underexpose by the 2-stops you want (so if you rated the stock 2/3-stop slower and then underexposed the scene by 2-stops, technically the negative is only 1 1/3-stops underexposed.)