When coming up to a release it's important to track the stable branch of a project so that we're actually running what we're going to release and can put our focus on fixing any remaining tiny bugs.
If you build all of Plasma with kdesrc-build you can easily switching to building the stable release with the following command:
kdesrc-build --Branch Plasma/5.7 kf5-workspace-modules
To change back, simply emit the --Branch parameter.