Effect of Sifting Powdered Sugar on Royal Icing?

I don't sift my powdered sugar. I use 10X and there's no need to take the extra step in my opinion.

I know some people swear by it, but I don't bother.

I have started with sifting, now I don't do it anymore. There is no noticeable difference in my opinion (no more or less clogging, no crumbs, same consistency). Maybe if the powdered sugar is kind of lumpy it is necessary, but mine never is. It might make a difference for some people though. A lot about making icing is partly esoteric anyway

I always used to, but I don't anymore unless adding to a small bowl of RI to thicken it to use straight away - I have a mini strainer for that 

I have been sifting, but now I am going to experiment a bit. I am letting the mad scientist out of the bag to have a showdown with my inner baking spirit, lol. I prefer to sift because it's a fast way prevent potential clumps, and I like to use very small tips. Also, I can be sure nothing strange is lurking in the bag with the sugar, lol! I always need to gently break up a few spoonfulls at the bottom of the strainer, but it generally crumbles easily, almost falling apart.

I had a recipe calling for two bags of sugar and, the chef mentioned how important sifting was to aerate the sugar. I never realized how much of a difference sifting makes, but two bags would not even fit in my mixing bowl! I had tested it for size by placing the bags of sugar in the bowl, and there was more than room to spare for other ingredients. (Now I have another reason to start weighing my ingredients instead of measuring with a cup.)

So I got to wondering if all that air is, at least in part, the culprit causing all the air bubbles in my icing? Especially combined with the meringue, which already whips up so well. Now I am thinking of sifting it ahead of time, then letting it settle before I use it. I'm not sure if I could sift all my sugar and store it away to settle until I need it, or if it will clump.

I believe anyone new to royal icing, or trying a new brand of sugar, should consider sifting at least the first few batches. Clogged tips are such a pain, and seeing what's left in the sifter should give you a good idea if you need to continue sifting. Maybe some climates cause sugar to clump once the bag is opened?

Humidity will cause the sugar to clump. But even so, I've never sifted 10x sugar and rarely (never) get plugged tips. I think plugging is related more to the quality of the sugar. At least that's been my experience. I also always weigh my sugar and eggs for consistency from batch to batch.