We have gone from hobby to business and we would like to keep some of the green in our pockets. 100 are far more apt to have individuals that could be susceptible to coccideosis than 20 birds. Once disease breaks out you would spend a lot more on medications and probably end with weakened birds. You could also have an elevator mix medicated feed for you to cut costs. I have raised many fine batches of Australorps with feed that had no medication in it at all.
Coccidiosis is a disease of poor sanitary practice.
The implication is that you have a clean building, clean feed and water, and that you keep your bedding clean. Vermin must be kept out, and ventilation must be good. I have never fed medicated feed beyond two or three weeks, when it is time to reduce the protein level the first step. If you must feed medicated feed to preserve the health of your birds beyond this point, you probably have a sanitation problem that should be fixed immediately. Allow your birds to develop their own resistance to disease and germs naturally as you maintain good conditions for them, and they won't need continuous medication.