Q: Do my chickens need to be wormed, and if so, what schedule should I use?
A: If your chickens have worms, you will want to treat them. Some signs you can look for at home are pale combs, a drop in laying and watery poo. However, it does no good to worm your flock--even on a seasonal schedule--unless you know precisely what type of infestation they are suffering from.
Keep in mind that particular wormers are only useful for particular parasites, so it is best to get a diagnosis as to which you are addressing. That way you will know which wormer will help their condition. Otherwise, you simply may be stressing their systems out by giving them a medication that does not treat the issue they have. For instance, tape worms and round worms are treated by different anthemintics (wormers), so if your chickens have tapeworms, using a wormer for round worms will not address the problem at all. And worse, it can conceivably make your birds sicker from the stress of medication they don't need. You will not want to worm your flock at all unless they actually have worms AND you know which parasites you are treating them for.
Your veterinarian will be able to perform a fecal smear and tell you what parasites your flock may be suffering from. Avian vets can be hard to find, but in many cases, any vet can perform a smear for you, whether they treat birds/chickens or not. Extension agents will sometimes make this service available, as well, and in most cases, you can find someone to do a smear for a nominal fee.
If you find your flock has an infestation, then you can treat them with the right anthelmintic; your extension agent or vet will be able to recommend the best medication for your flock's particular issue. If your flock is clear of a significant problem, you will know that if your birds are showing signs of illess, it must be due to a cause other than worms.