I don't breed, but if I do get the reason behind culling. I think it would be easier to do when they are small and can easily be fed to something else. I think that culling once they are fully grown would be harder, especially if it was for something like the fish not having a full spread or not enough rays. Though I also realize that it is a fish eat fish world in nature. When you think of it that way, and when you think of all the fish dying in cups at Petco, Petsmart, Walmart, the various dollar stores, in people's homes and offices in vases, or simply flushed live down the toilet because someone got bored with it, culling is definitely the most humane option.
Remember that the tank your fish lives in is his or her's whole world. Ask yourself this question, and answer it honestly: would you choose to live there?