Does one really need to add sodium bicarbonate if one already uses baking powder?
If the recipe calls for both (and it's a good recipe), it actually needs both. A common reason is that the baking powder provides leavening and the baking soda helps neutralize a bit of the acid.

It's a little surprising you're finding that baking soda ruins the taste and not baking powder; baking powder contains baking soda. I wonder if "ruins the taste" is an exaggeration or if you had a bad experience with baking soda and are a little more sensitive to it now. But if it's really ruining the taste, it's just a bad recipe. There are a couple possibilities:


the total amount of chemical leavening is too large in the recipe (it doesn't matter whether it's soda or powder) - make sure teaspoon hasn't been changed to tablespoon by accident!
there's not enough acid in the recipe to react with the baking soda so you actually have baking soda left over