@jacksmack:
Exid provided the answer. I agree that the enchantment doesn't necessarily restrict you to rerolling during the roll dice step, but it is restricted from affecting anything that occurred before it was revealed. Thus, it must be revealed before the Roll Dice Step in order to have any effect on it.
Besides that, it is still unclear whether the actual reroll happens during the Roll Dice Step or if it occurs immediately after the step. It is a very fine distinction, and shouldn't usually matter, but it is still somewhat unresolved.