I wait until mine are about 6 weeks old. Especially when they start eating "real food" because they can tend to be "little piggies" and get it all over.... I also make sure that they are completely dry and run my fingers through their hair to make sure that they are dry next to the skin. Alot of times they look dry on the top of their coat but when you run your fingers through their hair, it feels damp at the base of the hair.
Jan