You can easily skip the baking soda if the daal is soaked for about 4-5 hours. Urad daal gets fluffier while grinding. However, if you are looking to make very spongy, fluffy appe, ¼ teaspoon baking soda will help. Mix it well just before putting the mixture into appe pan. I have made appe both ways adding and not adding baking soda and frankly, they both have been very good in texture.