Always lock, no ones wants to see you on the toilet. If they do you need to stay away from that person... Even when im home alone since its force of habit, plus it would be embarressing if someone came in the houe while I was in there and I couldent lock it.
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.