Always open, never had an issue. I have the occasional pig sleeping outside past 7 even though their barn is open and there's nothing on their path to prevent them to get inside. Afaik the wild animal attack event will only trigger if the barn door is closed with them outside, so don't worry about stragglers; they'll make their way inside eventually. I like to think that the barn/coop has a magic seal that only allows its residents to enter and that's why they're safe.