Currently a stable release is created once a year. We would like to increase the pace to once in every quarter, but for that we need to simplify our processes, which is currently we are working on.
Nightly versions are stable. Saved data should be always backward-compatible: you can always load a scene to a newer version of Slicer than created the scene. There may be regressions in the nightly that takes a few days to detect and fix (maybe once in every few months), but these regressions very rarely can result in data loss.
If the stable build is older than a few months, I would recommend to use a nightly build. If you download a nightly and have no problems with it, keep using that for a few months (until there are some fixes or improvements in the nightly builds that you need).