meh, on my AR's I just leave it...
It won't get dusty in the safe, and it won't stay closed on the range, so what's the purpose? If I was draging it through the bush/jungle, I'd close it religiously, but for conditions we can legally expose the firearm to, it's not worth the brain fart to remember IMO. There is ZERO harm in closing it, it just doesn't have any value for us.