I don't mind either. I personally prefer using the black background whenever inside. It's like a "fog of war" kind of effect. To put it simply, the player can't see past the first tile of the wall so why map extra beyond that. Also, it's sometimes easier to use atmosphere for caves when it's black considering it makes the maps appear darker naturally. It's really just opinion though.
It looks stupid if you do that in caves.
When you do it for the inside of buildings, it looks fine because the ceiling tiles represent walls, and the black space around it is the outside/open space.
If you do that in a cave, it gives off the illusion that the cave walls are really thin, and there's space behind them.