Last year the week starting with Christmas had about six consecutive lows below freezing in Austin. If you are growing in something like that, heat might be helpful but it depends what's at stake. If it is just two strawberry plants, you might lose flowers, lettuce and broccoli on the other hand might get through ok as long as the nutrient doesn't freeze into a block so it all depends.

I just use a glass aquarium heater in the res at its lowest setting which is overkill, and 9a lows (virtually identical to San Antonio TX) are barely better than zone 8b (Austin TX) and could be worse on an off year.