Do you right down their defenses as a DM?
In my games I ask flat out, "Does a 17 hit your Reflex?" and allow the player to respond. I explain to them that must declair the immediate before the next player takes action and they loose out if they forget. I have had that Shield spell save the party Wizard once so far and it was because I announce the AC I hit. I don't see a need to hide my number from them, unless its the defenses of the monsters.
I guess my real answer is to give the dog a bone and let them know what you rolled as your to hit, which makes it easier and fair in regards to using such interupt powers.