I'm with you, there totally has to be a balance. Though I'd describe myself as someone who's more like your husband, where my door is always open, I myself always call ahead and never simply drop in on anyone!

I've always lived in big cities so it's never been a thing really where people just show up at my door (well, that's not completely true... when I was in university I lived on the main floor on campus and friends would bang on my window all. the. time. lol!), but I was raised to call ahead as a matter of politeness.

My office for example... I literally leave the door open unless I'm in a meeting or in a situation in which I actually need privacy (like a phone call, etc), and in those cases I expect people to knock. Though there are the occasional people that don't (ahem... this one executive at the company... ahem), most people know that a closed door means knock first! :P