A sorcerer's metamagic option Empowered Spell states (emphasis mine):
When you roll damage for a spell, you can spend 1 sorcery point to reroll a number of the damage dice up to your Charisma modifier (minimum of one). You must use the new rolls.
You can use Empowered Spell even if you have already used a different Metamagic option during the casting of the spell.
Do I have to announce that I want to use this metamagic before I roll damage, or after I've seen my rolls? The latter would be preferable, obviously, but the phrasing isn't 100% clear (at least I don't think it is, but then again I'm not a native English speaker).
Note that this is not a duplicate of this question, which is about before/after the casting, while I'm asking about before/after rolling. Comments to its answer relate to this question, but don't solve it.