Should we be updating copyright years on branches?

[Jonathan Wakely]

I just realised that all our source files still say YYYY-2013 on the
4.8 branch, because the branch was created last year and the script to
update all the files in January only gets run on trunk.

Is that a problem, or is it OK for files on the branches to only have
copyright dates up to the point the branch was created?

http://www.gnu.org/prep/maintain/maintain.html#Copyright-Notices
doesn't say anything either way about old branches, which would imply
the copyright year should be updated when making any non-trivial
changes on a branch, as for trunk (except we do it proactively in the
new year for the trunk, not after changes are made).