Hello,
in most of the cases when doing a release, I let Victor Seva do the tagging (if he is available) because he can also trigger the build of deb packages.
However, once the announcement is out to sr-dev/sr-users mailing list, backports can be pushed to the corresponding stable branch.
A tag can be created on a commit id, so that can be done even later, after new commits made it to the branch. Typically the tag is created on the last commit to ChangeLog file before the release announcement.
Cheers,
Daniel