When I am cohabitating with someone, be it housemate or boyfriend - bathroom door gets closed. Just not that interested in sharing that particular biological function.
When I've lived with housemates, it depends on the situation. When I've had female housemates where we kept the same hours, I would leave my bedroom door open unless one of us had an overnight guest. Easier than getting up every few hours to let the cats in (or out) of my bedroom. In mixed households, door stayed closed.
In my current situation, living with my somewhat unpredictable ex-bf, we each have our own room so I tend to close my door when I go to sleep (since he is prone to manic episodes where he stays up for 24-40 hours at a time and is well.. sorta drunk and crazy at times).
When I've lived alone (which I prefer), doors are left open.