We've talked about methods for sifting flour (How can I make sifting easier?), and the purposes of sifting (What is the purpose of sifting dry ingredients?).
How can I tell whether a recipe specifies sifting for aeration? How can I tell when to bust out the food processor? In what kinds of recipes will I see a better result by fluffing those dry ingredients?
For example, when I make cookies, I do just whisk the flour, salt and baking soda together in a bowl, and they turn out just fine.
Cakes are where sifting can make a significant difference. See Shirley Corriher's BakeWise book, she goes into it in detail.
Generally sifting is really only necessary when you are dealing with an ingredient that clumps a lot (cocoa powder, cake flour, and baking soda come to mind). For the average cookie or quick bread recipe, whisking these ingredients in a bowl before adding the wet ingredients will usually do the trick.
Any recipe that requires aeration of the batter (Creaming method, sponge cake method, etc), it definitely helps to get out the sieve and give the dry ingredients a good sifting. With the ingredients listed above, you will definitely noticed clumps in your final product or a denser texture. When it doubt, it never hurts to sift. I recommend using a small sifter with a handle for small batch recipes over a piece of wax/parchment paper.
sifting is always a good idea. while flour generally won't cake from ambient humidity due to anti-caking agents, you will get drops of this and that in your flour from time to time--good idea to get them out. in addition, it helps fully separate all the granules, allowing for better gelation of the starch.
According to Cooks Illustrated:
Sifting flour or cocoa powder is a chore, but sometimes it is important. When making a delicate cake like a sponge cake or genoise that requires flour to be folded into beaten eggs and sugar, sifted flour can be added quickly and distributed evenly (because sifting aerates the flour), thereby reducing the risk of deflating the batter. Recipes with cocoa powder, such as chocolate cake, also often call for sifting the cocoa powder. In this case, sifting breaks up small clumps of cocoa that form as the powder sits in the package. Sifted cocoa can be evenly distributed throughout a cake batter; with unsifted cocoa this isn’t always the case.
