Never. Not only would it be just plain, well.....mean, but it has a huge, and usually negative, effect on the flock. There is a very informative article about it in an issue of Backyard Poultry magazine that talks about not only the ethical and emotional reasons, but the scientific reasons to not cull. I cannot stress enough how HUGE of an effect it has on the rest of the flock, especially if you have chicks. I'm not so concerned about egg production, but still get more than enough eggs to eat and sell or give away, and even the oldest hens lay sometimes. More importantly, they contribute a LOT to the 'culture' of the flock, and keep the younger chickens in line. They become more experienced as they get older, and probably pass that on to their young. Taking them out the the picture would be totally counter productive. Roosters get really smart as they age. My oldest rooster, who passed away a while ago, was the most amazing rooster I've seen yet, and I wouldn't kill him for anything. How could I kill the older hens either, when they ALL have such amazing and unique personalities?
My advice: if you aren't running a commercial egg business, do NOT feel pressured to cull.