The packaging on flour often says that it is pre-sifted, but Art Smith says don't pay any attention to that! And, he says he always re-measures his flour after sifting. It does make a difference.
Don't use measuring cups for liquids when measuring flour either. The measuring cups for dry ingredients will give you a more accurate amount. Baking requires exact measurements for the best results. Always spoon the flour into the measuring cups and skim off the excess with a knife.
Art says many people in the South sift their flour three or four times to make sure the flour is light. Light flour makes light biscuits.