At least in the households that I've lived, if the door is closed and the light is on, it's in use. If you're still not sure, just knock. So I don't typically feel the need to lock the door at all, unless I'm in someone else's house or under special circumstances -- for example, the back bathroom is typically kept closed to keep cats out, so if I have to use that one (which is rarely) then I'll lock the door, just in case.
Always, because I fear being on the toilet and then that's the time when a dude with a gun comes busting in and I'm at my most 100% vulnerability. I'd like at least the protection of the door not being so easily accessible to give me at least a little time to react.
I know, I'm paranoid.