Necessary only if you notice diarrhea, worms is the stool or have a sick chicken. When I spoke to a vet a cornell, he said that whenever a chicken comes in sick they deworm it before anything else. I have to deworm mine, just a matter of personal preference. Everything under my care gets dewormed (except my DH), it's just the way I have been taught to handle all species. Once you see a worm in the stool, it is necessary to continue to deworm, indefinately. The eggs will lay dormant in the soil for a long time and the chickens WILL get them again.
I would definatley deworm for cocci when they are pullets/cockerals (technically not a worm, which requires a different medication than roundworms) cause you will never see those in the stool and they can cause some damage over time if not taken care of.
did that help?
mostly it's personal preference, just like I don't believe that any homeopathic remedy will work, with enough efficacy, to deworm