Hey there! I haven't been on BYC much, but this thread certainly caught my attention.
I never cull. There is no need to cull, nature does it for me. When I toss my birds, I might lose one, which I hate. When I fly my birds, a hawk may nail it, which I hate. When I keep pigeons, they may get sick, and the weakest die, which I hate. This all sounds cruel, I know it does. BUT let me assure you! I love my pigeons, and they get the top of the line care. I am a young person that can easily see myself raising the same line of birds for 50-55 years. The more I lose now to natural causes, the less I will lose in the future to natural causes. Survival of the fittest is played out in my loft. I try not to medicate. I give them healthy conditions to live in, so they can stay healthy. When a bird gets sick, I try to refrain from treating them. They need to build up their immune system. For me, a bird that doesn't slow down the group, doesn't get sick, breeds well, produces good young, comes home promptly from tosses, flys well on loft flys, etc has no reason to be sold. I lose the ones I could have sold, but you cant tell which birds won't be good until they are out of your reach.
This year, I am going to separate my pairs March 1st. Hens in one coop, males in another loft. On April 1st, they will be joined back together. All this does is ensures excitement in the pairs, and them to all get started in a nest at the same time. This way, I have a batch of youngsters all within a week of each other, which helps training them. I will breed out of 8 pairs, so if all goes well, I will have 16 youngsters at the start of the year, the pigeon year, that is. I then will breed a second round out of the same pairs to give me 16 young ones to sell and give away to promote the hobby.Then throughout the summer, I breed out of new pairs and color projects, just for fun and reference for next breeding season. I always keep those birds as I need to know if the pairing produced racing homers, or pigeons. Haha.
As far as culling flying birds that do fine (the majority of my flock today) I rarely do. I dont like to sell unsettled birds, as that then means the person who bought them won't fly them which then means they are prisoners. I don't like captives. But, I do sometimes have to sell them, because of overcrowding. Right now, I have 65 or more pigeons. I like to stay under 50.

Am I overcrowded? Yes... Is hawk season here? Yes... Will I be flying them through hawk season, accepting the loss of a number of pigeons? Yes... I like to go into hawk season with to many. I usually come out with about 10 less. Sounds horrible, I know. But when they leave the coop, they know the risk. They have seen hawks. But guess what? They still rush out for FREEDOM! If they could talk, they would say "Give me liberty or give me death!".
Sorry for the long answer. all that to say, no. I don't cull. I accept natural losses. Others in my area that I know may cull. But to be honest, that's okay. I have seen their setups, and their birds get good care. I would rather them eat their pigeons than the inhumanely raised chickens in the store. I can't quite bear eating my pigeons though, so I always sell them if I have to.