Really, if the ability that needs to be stunned falls within a DRG's stun rotation, the tank should not be stunning it. Tail Screws in Sastasha Hard come to mind. (Only time you're really using PLD stuns is if you're going for stun duration specifically. T5 Deathknights come to mind there, and that falls out of the whole "within DRG's stun rotation" bit.)
Both Dragoon's stuns are off GDC and do not interrupt rotation, something that PLD does not have and a Warrior has a long cooldown on. Plus, having the Tank pass off prey targeting on a DPS slows DPS and enables more Tail Screws to occur, which is bad, as you get 3 stuns before its immune and than you're entirely dependent on your healer to be flawless.
Tank eats the Prey Stuns, DRG stuns Tail Screws targeted at Debuffed Players. Usually the boss dies regardless before the 4th opportunity for stun is needed.
To Dragoons: You use your stuns on rotation unless the specific fight calls for it. Most bosses are outright immune, most trash mobs will die before diminishing returns matter. Every other time, you co-ordinate with your tank
Example: Diabolos
Nightmare can be stunned either by Tank or Dragoon. Doing so on Dragoon means you hold from your rotation and stun as needed and save the Tank the need to use them. Regardless, a Dragoon will have opportunity to blow both stun cooldowns when Diabolos puts up Noctoshield. (He becomes immune to stun and it the skills do not effect his building resistance)
The key to any of this is communication. There is no absolutes. Certain fights will always be exceptions one way or another. Don't be a pain to your fellow players, learn to work WITH, not against your player's advantages. If that means Paladin bows out on the gratification for a shield bash so the Dragoon can Leg Sweep/Spineshatter, so much the better, the PLD will get to continue rotations instead of interrupting them. If the goal is to burn a group of mobs down with AoE, and the group is confident it can down them before depreciating stuns are a problem, WHM can go Ham.
Take the time, be social, communicate positively. And the game becomes so much more enjoyable.