You'd want to use lower values for surfaces with little lighting difference, not specifically only in the 3D skybox - those surfaces are smaller and there's less of them usually so it's not really a significant gain to do this in the 3D skybox imho.
Lightmaps aren't very expensive compared to shaders and more advanced effects anyway... but saving on their resolution can have a significant impact on the maps filesize.
Also, what looks good is testable...