if you can control the lighting, you can light normal and grade in post(like day for night). if grading in camera, do a few tests to see where the skin gets grainy. there may be some youtube videos with similar exposure latitude tests called +1, +2, +3, -1, -2, -3 and then re-corrected back to normal to see how much detail is retained. Also, when shooting dark scenes its even more important to have high CRI lights as high ISO will introduce noise and loss of chroma color.
The best IRE will depend on the gamma camera settings. Each camera is different so a 10 min. test will save you hours of work.
Whatever you want it to be for the scene. Narrative is different from documentary/eng/beauty/commercial. Lighting can greatly effect how the scene is perceived, so if you're trying to keep someone's identity hidden, or hide the backdrop as they're lit by the light spilling in from wood slats, you're going to make exposure decisions based on THE WHOLE SCENE, not just the actor's skin IRE level.
Put it this way--if the scene calls for you to shoot the actor in silhouette, are you going to expose for skin tones and change the exposure in post? No way!