The only thing watering grass does, anyway, is create a shallow root system, rather than letting the roots grow deeper to subsurface moisture. The only time I've ever watered grass is when I'm trying to give freshly sown seed a chance to set roots before the birds eat it all, and that was only after a rather major excavation I didn't want to erode before a stand of grass was established to stop it.
Rain sensors are required now. Turns it off in storms as it collects water.