After you set up your hierarchy, regularly go back to look at how Conversations are getting categorized. Ask yourself this question: "Are Agents applying the right topics? Why or why not?" This will tell you if you have missing or unclear topics.
Other things to think about when you maintain your Topics are:
If you have “Other”/”Misc” topics, check those Conversations regularly to see what they’re about. Are they truly miscellaneous, or are there patterns that indicate you need to add new Topics?
Check for Topics that aren’t used. Is it because no Conversations match that Topic? You may be able to archive. Is it because Agents are confused about what it should be applied to?