Usually, untagged questions need tags. However, some untagged questions are closed and some closed questions have bad tags. Retagging would presumably bump them to the front page. (I haven't tried, lest it anger the elders.)
Is it alright to retag closed questions?
If they're useful to the site still closed and adding tags would improve the ability for them to be found, retag them. If they're complete garbage questions, vote to delete.
You can edit only the tags, but it still marks the question as "active" (I tested it). If you mouse over just to the right of the tags, you'll see the following option:
It would be nice if editing the tags in this manner didn't cause the question to marked as active, since it would be a good strategy for cleaning up/improving search results. Perhaps that's something the SE devs can implement.
