I wait for backports to be merged before closing.  Someone has to approve them or manually merge them before that happens, so leaving issues open until all merges are done prevents totally forgetting about them.

For doc issues, the patch, if not drastically wrong, becomes the new status quo.  So I think any further proposed change should normally be a new issue.  So once the backports *are* done, you can close.

For code issues, 'drastically wrong' usually means that the patch failed on one of the buildbots, or fairly soon somewhere else, and needs either reversion or a hot fix.