Wouldn't it make more sense to have a constructor param that allows you to pick any axis? (or even an arbitrary angle?) The reason I say this who is to say what way is up? or what what way they want their planet to be rotated? (or other sphere). Just a thought.
Well, with homogenous textures is makes no difference, how the Sphere is initially oriented. But if you have a texture with certina up and down, you will certainly want your Sphere to match it. And in case of planetary Spheres you will certainly want to ratate around the axis through the poles.
But if I really have some spare time, I will look, if I can add some constructors.
Marvin