I wish I'd bookmarked the post I read on here about someone whose feed consumption dropped dramatically after worming, because the worms were getting so much of the nutrition that the chicken consumed. I don't believe it's possible for chickens not to get some number of worms from soil and bugs; how many and whether there is enough to cause them a problem is another question. In people, it is certainly possible to get a negative fecal for worms and still have worms present. Actually, 3 samples are routinely collected, not one, for this reason.
Worms are going to make the chicken less healthy, possibly less productive, and they do damage their intestinal tract. Evidently pumpkin seeds decrease the count a bit, but I've never seen any scientific evidence that cayenne or DE do. And worms CAN kill a chicken, at least according to some posts on here.
Ivermectin is a great drug that also kills biting lice and mites, but, unfortunately, does not kill tapeworms. For that you need Valbazen (albendazole) which also kills the major other types. One program is to alternate the two, giving one either every 6 months or every year.
I eat my eggs after Epinrex, too.
There are also a fair number of people who have kept chickens for years and never worm, and will tell you their chickens are healthy. In the end it is a personal choice.