If the value had to be hard-coded instead of an option, I would start at least 5 degrees above freezing since the temperature may vary from the location point as well as the temperature may continue to fall while running. Some rain sensors can also include freeze detection, so checking a couple of their specs they too seem to like 37 degrees as a cutoff point.
What also would be nice with the location known is that one could work off sunrise/sunset times to identify the right time to begin a watering program. There is a C program called “sunwait” that is an example of this.