Does IV matter for pokèmon used for prestiging?

IV doesn't matter on prestigers because you won't need to max out it's CP. Move set is the most important for prestigers and attackers in general.

IV affects CP value, but doesn't always affect damage dealt, due to rounding.

IVs are the least beneficial factor determining a Pokemon's combat effectiveness between them, the Pokemon species, and moveset.

The only thing that matters is IV distribution, not total IV count.

For example, if you dodge everything including quick attacks, having a 33% IV mon with 15/0/0 will be more effective and beat your opponent faster than a 33% mon with 0/15/0, or 0/0/15

This is b/c when you dodge damage is reduced by 75%, technically reducing the effectiveness of both Def and Sta by 75% as well.

But really there's no need to micro manage it. Just something to keep in mind if you have a few identical Pokemon at similar CP values and are trying to decide who to keep.

Well, the answer is "yes and no". In your example above, IVs matter in that you can achieve a very similar effect with a Level 18 Golduck than with a Level 21 Golduck because the IVs are much better for the Level 18 Golduck.

IVs dictate the quality of your pokemon at each level (and its CP & HP). If you are shooting for a precise CP level (which is what matters most in Prestiging), you can reach it faster with a high IV pokemon if they start from the same level and you are powering them up.

But if you find yourself with two pokemons of roughly the same CP but different levels / IVs (like your example), then there won't be much difference.

An extreme example where IV matters for prestiging is the bubblestrat.