Just be conservative about the underexposure, you can always make it darker in post. Fincher often uses advanced noise/grain reduction software created by Lowry for film restoration work -- on some of his earlier movies shot on the Viper, I heard that he would de-noise every image and then add a consistent layer of grain or noise back in for the whole movie. Most of us don't have that luxury.
As Albion suggests, it is not the underexposure that is the problem, it is the corrections to the underexposed image in post, especially if you have to lift a particular channel. If underexposure itself was always a problem, we could never have someone where black in the scene or turn off the lights in the scene, etc. So at a given ASA rating, there is a base noise level in the blacks that you have to be aware of, because with a dark image, more of the detail will be in that area, and then there are the compression artifacts from the recording of that image. Both of these may limit your ability to make corrections, so my suggestion is to first test to find the optimal settings for ASA and compression level (if adjustable -- i.e. use the least amount of compression possible) and then expose as you see fit, realizing that if you are going to err, err on the side of not underexposing too far.