@ Callaway,
My dad has the locking doors at night ritual, like your dad had. Where they live, most of the people that are originally from there have their backdoors open, and, when you are known to them, you'll come in through the back-door, announcing yourself when you do.
My direct neighbour was like your grandmother for a long time.
I lock my doors at night, and when I go away. Most people will ring/knock at the front door here. Only two friends of my daughters do use the back-door. And, the brother and stepsister of one of those friends. But, they are not following protocol at all.
I like to have the back-door open. When away for just a short time, knowing my girls might come home in the mean time, I often do not lock it either. At night, I do want to have it locked though.