Depends...there are predators everywhere, even in town. Coons, possums, rats, weasels, martins, bobcats, even snakes! I definitely would have a door that could be closed and locked (coons are VERY handy with their paws!) I don't mean with a key, but a secure latch of some sort. Not to mention owls, cats, dogs. If you are willing to face the possibility of losing every chicken you have...then sure, go ahead and leave the coop open. You will probably get by with for a while (how long depends on where you live) OR, you could lose them all in one night. Just read a post on here earlier about that happening to someone who left their chicks in a chicken tractor the very first night and something got one of them. So, you decide...to help you decide, type in predator in the search box and read some of the horror stories of chickens with their heads or wings ripped off or completely missing, or just killed and left.