In each Ubuntu Kernel Team git repository the Ubuntu-* tags represent the git commit which was used to build the official source package for any particular upload. For instance in the ubuntu-precice git the tag Ubuntu-3.2.0-23.36 represents the 3.2.0-23.36 source upload. Some tags have an additional -lts- component indicating they are the official LTS backport for the containing release, in the ubuntu-lucid git repository the tag Ubuntu-lts-3.0.0-18.31 represents the 3.0.0-18.31 oneiric LTS backport for lucid.
You can use the Launchpad versions page for the linux package to determine the current versions uploaded into each pocket of each release:
All recent tags should be signed by the committer to prevent modification. You should be safe to build any official tag and expect to get a substantially similar result to the uploaded binary packages for that release.