Adding baking soda is sometimes done to prevent any curdling. Tomatoes are, by nature, acidic, and high amounts of acid can curdle lower-fat dairy (such as 2% milk). Even just 1/2 tsp of baking soda will work, so if you are using a lower-fat dairy milk, it's an option if you really want to be on the safe side. I promise, you won't taste the baking soda!