I vaccinate for PMV. I do not give any medications until I see signs of sickness. I think many people way over medicate our birds as a preventative only to have them die because they have built immunity to the meds. I think the key to having sick birds is vitamins (Winsmore), apple cider vinegar and garlic. Do not feed your birds in the loft. I feed in the aviary away from the floor. Make sure water is clean. Once again I do not have the water in the loft. Keep things dry and clean. Some people use wood shavings. If the shavings are not kiln dried they can promote ecoli, I prefer to use pine needles.
Appart from vaccinatiing for PMV or pox, there is no preventative medicine that should be given.
As JSummers has said, if you medicate when not needed the birds build up an immunity to the meds and then when they are needed they are of little use.
Clean loft, good food, clean water every day, remove excess food from loft, probiotics and acv and you should have healthy birds.