I’ve developed the habit of using the revision summary to indicate which changes I’ve made to the tags while retagging a question.
Edited tags: -cocoa, -touch, +cocoa-touch
which I find useful (and potentially educational) for the OP and other editors.
On the other hand, when an editor doesn’t type a revision summary, Stack Overflow shows
User (timestamp) edited tags
with no indication of the difference between previous tags and the tags set by that edit.
Since there’s a limit on the number of tags, this difference would easily fit in the revision history list of that question. Could this be automatic in case the editor doesn’t fill the revision summary? Is it worth it?