I think the answer to my question is it depends on how granular you want to auto categorize.
Perhaps someone can answer when using automation to automatically categorize topics does the search input box being matched take priority over using AI?
In other words if I set Banana as the search term in automation and a user creates a topic using Banana within the post, Match, No AI called. Categorize as banana.
If the user creates topic which references yellow fruit monkeys eat, No Match, AI prompt runs and may determine that description is a banana 
.
For this scenario Banana would also be an existing category.