Pink eye seems to most frequently be something that happens when stock move on. We get this question repeatedly and it almost always starts with “I just bought some more sheep and...” I’ve had bought-in sheep have pink-eye when my own sheep had shown no sign, and the seller, who I knew, also had none, so my personal theory is that the stress of moving lowers their resistance enough for it to get the upper hand.
So I suspect that one good policy would be new sheep in their own field, with no adjoining boundaries to your own sheep, for 3 weeks or until two weeks after the pink eye has gone, whichever is later, before mixing them with your own stock - and that in a different field to the arrival field.
We don’t buy in here, hopefully won’t even be bringing tups on for the next two or three years, so I can’t test my new regime out. And I know not everyone has the luxury of completely segregated areas.
The “treat or not” question is also a tricky one. Bringing them all in to check and treat almost certainly helps the bugs to spread :/. Personally I let it run its course and leave the sheep running extensively unless I have a sheep which is clearly suffering, then I would try to catch and treat that one without bringing the whole group in. Can’t always manage that, of course.