It isn't a bad idea to raise the KH a bit with baking soda. But, the effect is to raise the pH a bit, not to increase the pH stability of the tank. KH does not stabilize the pH, once it is above zero - let's say 1 to be sure it isn't zero. Once the KH is that high the pH changes will be the same as when it is 10. I know this is counter to everything we read about KH, but it is still true.
Another reason to have a higher KH is that some plants can use carbonates as a source of carbon, driving the KH down a bit. Obviously that would be a bigger problem if the KH is very low, than if it is above 4 KH, for example.