The problem with attempting to preserve maximum data in an 8-bit JPEG is that the wide gamut you'd use (usually ProPhoto RGB) requires 16-bit colour depth to avoid posterisation when editing. Thus, creating such a JPEG wouldn't often have a purpose. Instead, you'd typically print straight from the raw processor or create a 16-bit TIFF. Of course, a wide gamut wouldn't save a badly overexposed image where all three RGB channels are at or near full saturation, but it does help in highlight recovery. Lightroom uses a version of ProPhoto RGB in its default histogram, and it'll often be the difference between channels clipping or not, which can be seen by soft-proofing smaller colour spaces. I wouldn't get too concerned about the 256 levels of an 8-bit JPEG histogram; you'll see the same histogram when editing 16-bit files, too, because it's more practical and intuitive to edit. The presence of the data is more important than its representation in numbers. I don't know how Nikon raw software works, but for maximum latitude you don't want a carbon copy of in-camera settings. The histogram on the back of the camera is inherently less forgiving than one representing a bigger colour space, especially if you also skew it with colourful picture modes. To get a bit nearer to the leeway of the raw file, it'd be best to choose the largest colour space possible in the camera (typically Adobe RGB) and use the most neutral picture mode. That's a very inexact method, but it'd probably be a bit more representative.