I am frequently asked by some WordPress users if they should categorize both in the parent and the child categories. Lot of people do it so that the parent category gets listed in the
the_category() call for that post.
In my opinion, there is no point in categorizing in the parent category if it is aimed for the child category. WordPress already shows the child category posts in the parent’s category archive. As for listing the parent category for a post, you can explicitly tell it to do so.
the_category(', ', 'multiple') will display the parent category for that post, even if the post is not explicitly categorized in it.
And if you are thinking that there is no harm in categorizing in the parent categories, there might be unintended side-effects.