Please don't worm your chickens "just for the heck of it" or "once a year whether they need it or not". If you suspect worms, have a fecal float done, find out if they really need worming, and pick the correct medicine for that worm. Unless you see one in their poo and it is really obvious. There are people on BYC who have never wormed their chickens. It can be hard on them, as well as contaminating eggs and meat. If everyone uses medicine when it is not necessary, it will not work as well when it is really needed (organisms develop resistance).