I don't know rules for LTS, but the semi-official (i.e. officially .org hosted but officially unsupported) backports.debian.org seems to a) recomend including all changes compared to newest parent release in topmost changelog section(s):
It is recommended to include all changelog entries since the last version on debian-backports or since stable if it's the first version. You should do this by passing "-v" to dpkg-buildpackage. Eg: "debuild -v0.7.5-2", where "0.7.5-2" is the version in stable.