I would think if I select a child category, that the parent category would automatically be selected (with no choice to deselect the parent), but it's optional.
Looking at how Wordpress sets up this user experience flow:
I've noticed if I create a new post, and assign it to an existing child category, it simply ticks the child category option but the parent category is not selected.
Alternatively, if I want to assign a new post to a non-existent child category, I would use the Add New Category function at the bottom of the category selector and type in my new child category name as well as select the parent category from the drop down. When I do it this way, it defaults to selecting/ticking both the child category and the parent category.
What is the best practice for this?