I grew up with doors open, but even then, everyone, and I do mean everyone, would knock first.
never had a problem
then of course, times have changed, until fairly recently our doors have always been open, but no one has ever just walked in.
Sharing is very difficult.. being recently, well fairly recently , living on my own for the first time in my life, I often "dream" of someone living with me, or me living with them, but I just know it would not work.. if it had to be,, well, I would ensure clear understandings, my home, my rules?
so I have two grown up sons, 4 gks, and 2 very close friends, all have a key, (just in case) not one of them uses the key!
everyone one knocks and waits,, as I do when visiting their homes,,, knock and wait, and I would consider us to be a close family.
manners? well !!!!