When reviewing post edits you have the option to Improve Edit as well as Reject and Edit.
These options are no longer available when a tag-wiki edit shows up in the queue (there have been quite a few recently). The reason for that is a mystery to me. I have been missing these options a couple of times.
As a way around so far, the only option to emulate the above seems to be either Approve or Reject (respectively) and then go and look for the tag to actually make relevant improvements.
Do we consider that someone making a tag edit is either spamming or careful enough to produce a perfect result? Or is that a bug?