I didn't used to sift my powdered sugar (I use domino, top quality in my area) and one day while decorating using a small writing tip then again with the leaf tip, small clumps of sugar stopped them up several times, and until it was in the tip and actually stopped them up I couldn't see the clumps, now I sift once every time I make icing, and have not had that problem since.
As to when to sift, I always sift before measuring, don't know if this right or wrong just how I was taught. Most of the time I weigh out my sugar, and don't measure it by cups, and it weights the same whether it is sifted before or after.
Wilma
NOTHING TASTE AS GOOD AS BEING THIN FEELS!
NOTHING TASTE AS GOOD AS BEING THIN FEELS!