Winter not too many predators. I don't worry if the door remains open on a rare occasion. We do get mice in the shed in the winter so I set lots of traps for them.
Spring, Fall, Summer...I know there are racoons in the neighborhood, we have also had skunks in the past, hawks are everywhere, and coyotes would eat the chickens in a heartbeat if they were the easiest prey of the night. Given that info, I am more diligent in the warm months about closing the pop door at night. And let's face it, I'm more likely to walk to the coop in the dark when the weather is nice rather than in the winter when it gets dark so early and it is way below freezing at pop door time.
So far, 3 years, this is working out ok.