I think the basic summary is this.

On mobs you know Stun isn't required or helpful - Stun to your heart's content.
On mobs that absolutely require stunning - Let the tank stun
On mobs where stunning is helpful but not required - Save the stun for the ability, unless you are sure you can burn it down before the ability comes out.

Did I get that right? I don't main a stunner, I honestly don't know for sure.