Go ahead and sift it before measuring to guarantee you get the right amount. Sifting flour over a work surface, instead of just tossing it, when you're about to roll out or knead dough is a good idea if you want a thin layer of flour, since adding too much extra flour to your dough can make it tough or dry.

Is sieving flour necessary?

Well, I think that sifting flour is often a waste of time. ... Now, most commercial flour is refined and clump-free, meaning there's no real need to sift it. (You should, however, use a kitchen scale to ensure that your cups of flour aren't way heavier than the recipe developer's.)

Should you sift flour before or after measuring?

If a recipe calls for “1 cup flour, sifted,” measure the flour first and then sift it into a bowl. If a recipe calls for “1 cup sifted flour,” sift the flour first and then measure. What sifting does is aerates the flour (and other ingredients) to make them light.