Should I sift my powdered sugar before mixing it or does it matter?

AI don't bother if its a new box.

AI always sift my powder sugar even if it's a new package. I just always try to play on the safe side...would hate to have a lumpy buttercream.

Great. Thank you

I would sift it just to be on the safe side, dont want to find lumps in your buttercream.

For American style buttercream only: If you think there might be lumps, first beat the sugar with your liquid. That should take care of any potential problem. Then beat in the remaining ingredient(s) and whip until light.