I usually handle it this way: On interior scenes I like keeping all the walls (incl. floor and ceiling) separate throughout the creation process of my scenes, simply because then I can just uncheck their visibility in the outliner depending on the area I’m working on and if they’re in the way. Makes it much more convenient in my opinion.
Then as a final step before the final render I sometimes join them back together, sometimes not. It highly depends on the visibility of the corners and if it would be obvious, if there wasn’t a small bevel in the corner in the final render. That I do on a case by case basis.
On exterior scenes I mostly keep them as one mesh, or sometimes separated by cladding type. That is if I don’t venture into the interior too much on the same scene. Again, like the others already said, it’s on a case by case basis.
