I do not treat anything except new arrivals. Depending on the source, new additions get between 30- 90 "days in the hole". I treat their tubs at least every 2 weeks, or when their tub needs cleaning. They get handled/fed/cleaned last. If they refuse a rat, that rat has its own QT enclosure as well for that "batch" of additions only. This was fun when I had 8 pick-ups from Daytona.
For this reason I decided to breed my own rats. I cannot depend on a clean source of rats from ANY pet shop. On rare occasions now I need a handful of medium rats for a few big girls. I hope to be fully self sufficient on rats by spring since feeding is slowing down now during the bp breeding season. I have 36 female breeder rats and will add at least 20 more by spring.