I agree with April‘s comments from a safety perspective (e.g. tornados). We luckily don‘t get those here, so I don‘t see a need for training him to go down there. The risks would definitely outweigh the benefits, and there is really nothing of interest to him the basement. Similar to Boxergirl‘s comments, most of these steps are very steep, which makes it even challenging for us to go down there without risking any accidents.
I would personally keep them upstairs unless there is a real need for them to go downstairs.