The most recent official firmware is 2.1.7. If 2.1.2 works fine for you and you don’t need any new features introduced in later firmwares, you don’t have to upgrade. When you upgrade firmware, there is always something that works differently than before, so if you don’t want any surprises, you may want to wait till it’s not the high watering season to update firmware.
The firmware release notes are all here:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/releases