You probably won't get your site structure right in one go unless you have got a really simple, really static site. Is that a bad thing? No, it isn't—because websites evolve and Joomla makes it easy to start with a provisional structure and change things when needed. Maybe there is new content available that has to go into a new category; or maybe because when you are actually adding content, you learn that your well-organized site isn't altogether logical after all. That's fine; keeping a close eye on the structure of your website is a continuous process. Luckily, categories, once defined, can be changed easily without any consequences for the articles they may contain. You've seen how easy it is to add new ones, and it's equally simple to move content from one category to another.