There are many sites and forum posts which suggest that you need to edit Joomla! CSS or PHP files in order to make the text "Subcategories" not appear when you want the content of subcategories to appear on, say, a blog page.

In recent versions of Joomla!, provided that it is supported by the template that you are using, this is not necessary. All that you have to do is go to the Category tab of the item and change "Show subcategories text" to "Hide".