When deciding to index/noindex a page I always ask myself: "does this page contains any unique content, content that can't be found anywhere else on the website?"
For tags pages the answer is always NO. On a WordPress site Tag pages contain either automatic excerpts and this content is also available on the actual posts pages or in category pages, either manual excerpts and this content is also available in the category pages. So I decide to noindex tags.
In your case, where the tags pages bring less then 1% search traffic I would noindex them without worrying.