Until now it has been possible to handle the same project with both 5.1 and nightly builds provided that you manually deleted “max_error” line from the pcb file which was put there by the nightly builds. Some new features are incompatible (chamfered pads, hatched zone filling) but if you don’t use them they don’t affect files.
Now it looks like the branches will soon diverge too much to be even semi-compatible. See https://lists.launchpad.net/kicad-developers/msg41324.html.