With tapeworms, it's not necessary to worm the whole flock, only birds showing segments in feces. Remember, a bird has to eat an infected insect. Not all birds eat infected insects.
However with nematodes, (all roundworms,) it's always best to worm the whole flock.
I prefer to cage and treat birds with tapeworms. It prevents the segments from ending up in the soil where they release hundreds of eggs and insects eat them and become infected eventually. After I treat the birds, I like to see the whole tapeworm(s) excreted. That way I know it's dead. I always reworm 10-14 days later too.