Problem is if you are in a fluke area you would probably need to treat now, as sheep are outside in winter but cows come home... difficult decision. How long do you milk sheep for? They do not run through the winter do they?
It is the same problem with goats, I have not yet had reason to believe that my goats have fluke, but I know others do. I would think the same applies as for wormers... minimum 7 days withdrawal for human consumption.