Drmambo23's answers are all 100% correct. If you read closely, you'll notice they don't say that it has to be revealed in response to any trigger, just that it has to be revealed before the creature is destroyed, e.g. before the damage and effects step, in order to actually function.
You certainly could reveal sooner than that if you wanted. But between the roll dice step and the damage and effects step is the absolute latest possible, because as soon as the damage is applied, the creature and the unrevealed enchantment both get destroyed, and an unrevealed enchantment has no effects.
On its own, you probably want to wait as long as possible before revealing. But you are absolutely right that other effects, like the Adramelech Warlock's Flame +1 ability, might give you a good reason for revealing sooner.
