I don't use a dewormer unless there are actually intestinal parasites, either seen, or in fecals done. Then, it's a question of 'is this actually a problem, or such a low parasite load that the birds don't care?'. There's only water soluble fenbendazole available for laying hens; other products are off- label and must be used carefully if at all.
Mites and lice and fleas are best treated with permethrin, and the spray is easy to use and very effective.
I've had more episodes with mites than with anything else, from the wild birds around here.
You can't 'prevent' parasites, unless your birds live indoors all the time. If they are outside at all, or if wild birds come into your coop or run, or if you feed certain raw products, your birds will encounter some parasite or other. Then it's time to act.
Mary