Yes, a week or two before they're due to come in to lay is good, to get them off to to a clean start. For me, it's part of the process of using the time between getting them at 16 weeks or younger, letting them settle down in the new environment through the quarantine weeks, and preparing them for integration to the older flock. I try to time it so that all the older girls get dosed at the same time, so they're all in synch. thereafter. They shouldn't be done earlier than 16-17 weeks and normally they wouldn't need it before then anyway, if raised in decently clean conditions.