I would recommend to disable tags indexing as there are cases where you are multiple tags for same topic. You can index categories. They are more structure and define your website in some way. If you write custom excerpt for each post, it helps categories to have unique content for each post except.
Read this article for example:
WordPress Categories and Tags - Which to Use and How?
Learn how to use WordPress Categories and Tags effectively on your blog so your visitors can find the information they are looking for.
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