Do I need to sift flour that is labeled sifted?
Maybe! 

Flour is unusually variable in how densely it will settle, so this can make a big difference for some recipes. The purpose of sifting is to make the amount of flour in a given volume reliable. (If you are measuring by weight, you don't need to sift.)

By moving around the sifted flour, or pouring it from one container into another, you are changing the way it is packed. Therefore, you can easily "unsift" it in the course of normal handling. Proper sifting is done straight into the measuring device.