This is a good question. I live in a warm area of the country and worry about internal parasites in my chickens, but I have never wormed them and have been keeping birds more than a decade. I have had very few unexplained deaths in that time, and none that I think were probably parasite related. My birds have never exhibited any symptoms associated with mites, either.
The key is to give them high quality food, lots of exercise, and large clean pens with low population densities of coop mates. My penned chickens are kept on six inches of white clean sand with hay placed over it. I change out the hay every couple of months. The manure seems to percolate into the sand and there is never a bad smell or mud.
I also feed the birds diatomaceous earth mixed in with flax seed. I've read that on a microscopic level the diatoms slice into intestinal worms, killing them. Who knows? It doesn't seem to hurt the birds, and so I do it.
Anyway, I sometime feel guilty because so many people seem to worm their birds, but then I look at my birds and they seem very healthy. So, I err on the side of not medicating them.