There is no situation where you must use baking soda, even when you have an acidic ingredient in your dough or batter. Because Baking Powder contains both baking soda and an acid, it will create carbon dioxide bubbles even when there's extra acid present, such as the buttermilk.
You can choose to use Baking Powder completely, If you do, the flavor of the acidic ingredient (buttermilk, etc.) will be slightly more pronounced since there is no baking soda to react with or neutralize it. The texture will also be a bit finer than the coarse or “shaggy” texture that is characteristically caused by the action of baking soda.
You may find you like the flavor and texture of things leavened with baking soda or you may prefer Baking Powder. Try a recipe both ways. Just remember that you can’t use baking soda in place of Baking Powder without something acidic to react to it. Without something to neutralize it, it will leave a bitter, salty taste. And always blend either one thoroughly into your dry ingredients first so it will be evenly distributed throughout the dough or batter.