I generally let the players see my dice rolls so that they know I am letting the dice fall how they may. I don't tell them the modifiers, but they do see them, and they take them into account. Similarly, I let the NPCs have an idea of what the players' rolls were (though not the modifiers). Thus, if the Warblade rolled an 18 anyway and wasted the manoeuvre, the bad guys might think "She just got lucky--her willpower will run out eventually!" but if the Warblade rolls a 1 or 2 and succeeds, they'll think "She shrugged that magic off as if it didn't even exist! Either her willpower is like unto that of the gods or she has magical protection against that spell. Either way, not trying that again!"