Absolutely! We spray our lambs with Crovect (don't like Clik) in May, at the beginning of the fly season in our part of the country. We would be very upset if they were already struck at that time. We watch the adults carefully just in case (in which case we would treat straight away) then spray them at shearing in June or July. By hand shearing there's enough wool to hold the product, whereas with machine shearing you need to wait a couple of weeks. Sheep are unlikely to be struck when freshly shorn, as there's nowhere for the maggots to hide, or the muck to cling.
With Crovect, spraying is repeated every 6 weeks through the fly season, but we find it's best to do the repeat dose a couple of days before the six weeks is up.
Just because the sheep and lambs have been sprayed, it doesn't mean you don't watch them carefully.
Cross posted with Hellybee