It often makes sense to add one tag. But we require that you have [500 reputation](https://askubuntu.com/privileges/retag-questions) before you can re-tag questions. You also need [300 reputation](https://askubuntu.com/privileges/create-tags) to create a new tag.

 - Suggested edits should always be substantial improvements to the post.  
 You're encouraged to improve the wording, grammar, spelling and layout of the post as well. 

In this particular case, I think the planet-ubuntu tag is indeed unnecessary. The community tag covers all questions related to the planet, and the fewer tags we have, the better.