I would think that you'd never use the actual heartwood for woodworking... When I think of all my guitars and basses, that one neck is the only one where the grain is curved that way. All the rest are "bark side" toward the fret board.
The issue may not have anything to do with the harder/softer factor, maybe it is just with how the annual ring structure in the neck strengthens or weakens the tendency to bow one way or the other.
I'm not a luthier (and that forum would probably make hay of this) but have been a pro woodworker at many times over the years (look, Ma, still have 10 fingers
I've always found the theory that the location and condition of the bolt holes causing differential moisture conditions and possible movement at the bolt-on end of the neck leading to "ski jump" to be an interesting theory, but I don't know enough to know if it's accepted as fact.
...just my thoughts on this.