I cull.i don't use antibiotics or chemicals, for the most part and so if one bird shows a lack of immunity to parasites or an illness they get culled. Also, although I'm "temporarily" at a time in my life where I'm raising urban birds and stuck in the city, most of my poultry experience has been in the country. I've culled whole breeds out of my flock ( not necessarily killing them) because they simply weren't predator proof and I prefer to free range. Some breeds just aren't smart enough, fast enough or the right color to survive outside of a cage. Over the years, I've sort of learned what to look for in a good free-range breed but I'd did lose quite a few along the way, usually either white birds, foo-foo breds or breeds that can't see or move fast.
My one and only experiment with a dewormer, after 25 years of organically deworming, nearly ended up in losing three Fayoumi hens. Back to my organic system, which has always served me well. I have no idea why I decided to use a chemical dewormer after all this time except that everyone on BYC was using them and since nobody was laying at the time I didn't need to worry about egg withdrawal periods. Well, never again. Apparently, my breeds of birds prefer the natural methods better. I'll go back to not deworming and will cull if somebody appears to be infested with worms, which is really rare.
I use DE, pumpkin and squash guts, pepper seeds and just watch for anybody who may appear to be losing weight or have issues. So far, in 25 years, I've lost one bird to parasites, a Silky that I bought from a show breeder.That breeder's birds ( a group of different bantam breeds) didn't thrive in my program, unlike the stuff I raised and hatched myself, so, for the most part, I ended up getting rid of them, or they got rid of themselves because most of them got eaten by predators, not being the brightest birds in the flock. The only ones that survived, in fact, were Old English Game Banty's, a breed that I believe is quite capable of foraging, taking care of itself and very hardy as far as disease resistance. In fact, I currently keep an OEGB as my flock rooster, great little protector guy.
So, unless you want to keep a bunch of birds with crossed beaks, amputations and repeated infections, I highly suggest, if you're going to consider anything more than a small flock of 4-5 chickens, that you accept culling. It's simply part of farming and breeding livestock.
