The answer is: definitely sift if the directions ask for it or even if you are just mixing more than two different dry ingredients. The importance of sifting ingredients shouldn’t be underestimated. Many people skip sifting because they think it’s a waste of time, but actually sifting is very important because it can avoid many problems in your finished baked product especially in a cake.
First, sifting removes lumps from the flour, ensuring your baked item will be free of dry lumps of flour in the batter. Second, sifting better incorporates the leavener (baking powder and/or soda) avoiding uneven pocked marks in the top of cakes or other types of cake like baked desserts.
Get the kids involved by laying out some wax paper and let them sift for you!
Extra sifting tip: The best way to sift cocoa powder is by shaking it through a fine mesh strainer.