Every group I've played in or DMed has gone with "Yes, you can reroll after the DM has declared a hit or miss", though often the player doesn't need to know, as they're already pretty sure about the outcome (if you needed a 12 or better to hit the target last round, a 4 probably isn't going to do the trick this round).
I'm not sure there's an official rule on exactly when it has to be declared.
The most recent wording is that it triggers "When you make an attack roll and don't like the result.", which, depending on how you interpret result (Is it the number on the die, or is it whether the attack hit or missed?), changes the timing.