It often makes sense to add one tag. But we require that you have 500 reputation before you can re-tag questions. You also need 300 reputation 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.