Not the person you asked to, but I have a worm issue in my backyard and I regularly take my chickens to the vet, and here's what he said to me about this:
Most chickens have worms, especially if they free range as they can easily eat the worm's eggs; this is usually not a big problem as worms are parasites and, as such, they do not intend to kill their host. Many chickens worms actually live in their intestines and feed themselves from what the chickens eats.
The problem is that sometimes chickens can have a very high amount of worms, and they develop symptoms that some chicken owners can easily overlook, such as weight loss or mild diarrheas. By the time that we notice severe weight loss, frequent or severe diarrheas, or blood in the poop, etc., the chicken has a high infestation of worms, which may have produced damage in their intestines and poor absorption of nutrients - and sometimes this is chronic.
So, will it hurt if you don't deworm them? It really depends. If your chickens have worms but in a moderate amount, and the worm population never increases, perhaps no, it won't hurt them. But as the vet says, you don't want to wait for the symptoms to appear, you want to prevent the worms from settling on your chickens' intestines, or at least control their population. The vet prescribes meds in different dosages to prevent worms or to eliminate them if the chickens already have them.
What meds? It depends on the type of worm. Sometimes, you can identify the worm in the chicken's feces; sometimes, you can guess another type of parasites due to the symptoms (for example, if you see blood in the feces, you can suspect coccidiosis). Sometimes, you need to send a poop sample to a lab to be 100% certain.
Now, there are "OTC" medications that you can try on your own, and they're mostly safe and more effective than natural remedies, but as far as I know, none of them can cover all kinds of worms. Anyway, I never tried those so I can't recommend anything in particular, wait for other people's responses.
