Elevation is a less important attribute of Road types: it had no affect on routing.
In most cases the elevation should be defined "as is" in the real world. Standard "Ground level" or 0 or in case of a cross-over / fly-over the upper road should be defnied with an elevation one higher. In case of a tunnel the elevation could be -1 or -2.
Don't split segment to change the elevation for bridges. It is better to give the whole segment the same elevation.
See also the global Wiki page on Overpasses, Underpasses and Bridges.