I love it when they attach on the top in a topbar hive!
Mine have attached small portions to the sides but never to the bottom. I think they need their "bee space" to maneuver around the hive. In my favorite hive, they have built beautiful combs exactly the shape of the hive, only a tad smaller for their moving-around space.
I have not actually seen inside a Langstroth, but my understanding is that the bees move up and down the frames, therefore not mattering a bit if they attach tops of frames to bottoms of others. In the topbar, their horizontal movement would seem to preclude this.
Isn't this all fascinating?