People are worried about Inclusion Body Diseased (IBD), or whatever it is that the many of the IBD cases have turned out to be instead. It can take several months to show, and it has killed entire collections. That what the longer QT periods are about. You should search the threads on IBD for symptoms and more info.
I always do at least a month or two, because many snakes shipped to me have had internal parasites, specifically flagellated protozoa. The stress from shipping, especially in colder weather, can bring on a bad case in an animal that originally had only a very low level of the parasite, so the breeder never knew about it. Symptoms there are loss of appetite, hiding for weeks at a time without coming out even in the evenings, sometimes runny stools. Plenty of healthy animals act like that, so you need a fecal smear to know for sure. I've had to treat 3 animals for it, two of them piebalds.