The overnight lows are supposed to get down into the low 40s or high 30s later on in the week. While such temperatures will not kill the basil outright, the plants will be greatly deteriorated and virtually unusable. So I'm calling for a free-for-all on Tuesday. You can cut the stems as low as you want, and in fact, if you put long cuttings in a vase like a bouquet of flowers, the basil stays fresh for almost a week (basil lasts longer this way than in the refrigerator). The lower portions of the stems are pretty woody at this point, so you'll need either very strong scissors or pruning shears of some sort. Any basil that is leftover as of Tuesday evening can be used for CSA subscribers.