My Wilton instructor also told us to use pure cane powdered sugar as well. 10 X means that the sugar has been sifted 10 times. The powder sugar will be more fine and turn out better because it has been sifted more. You could buy the store brand and the pure cane and test it. I've just always used pure cane so that the icing will be extra nice. =o)
I was told that pure cane crystals are different than beet sugar crystals. For baking and things like frosting, the cane sugar crystals mix together better with the other ingrediants. You can use the beet sugar for other types of cooking, or for things like iced tea, where you dissolve it.
My Wilton instructor also told us to use pure cane powdered sugar as well. 10 X means that the sugar has been sifted 10 times. The powder sugar will be more fine and turn out better because it has been sifted more.
I respectfully disagree.
The 10x means it has been ground to a certain fineness. 6x is a coarser grind of powdered sugar, 12x is an even finer grind of sugar.
It has nothing to do with the number of times the powdered sugar has been sifted.
I buy both 10x and 6x. There is really no discernible difference, IMO.
Store brands are not always beet sugar. I have been using the Walmart brand for 2 years now with no problems. Just check to see if the ingredients say cane or pure cane sugar. If it does you won't have a problem.
(BWT, I'm a WMI and always tell my students that store brand is fine as long as it says cane suger.)
I thought about this at the grocery yesterday and looked at the bag for the name brand and the store brand-- both said sugar, cornstarch and the nutrition facts were exactly the same. So, how do you tell if there is beet sugar? I know the name brand calls out "100% Pure cane. contains NO BEET sugar but the store brand says "Confectioners 10-X" no mention of beets. The name brand has no mention of 10X or 6X...
Confused now!
This issue launched me into a quest for information last winter. I use the 7-pound discount store 10x sugar - Sam's or Costco - and have never been aware of any problems, but I am not "educated" in the science and art of baking like a trained chef would be. Research says that beet and cane sugar are virtually chemically identical and there are no differences in their behavior or taste. Those who use the sugar have a different opinion 
. I e-mailed the company that distributes the Costco brand, and their rep responded that they broker huge amounts of sugar, buying what is the best deal at the time. So, their sugar is almost always a blend of cane and beet. If it doesn't say "pure cane sugar", it will likely be a blend of beet and cane or just beet since beet is cheaper to produce. Check out this article:
http://www.sfgate.com/cgi-bin/article.cgi?file=/chronicle/archive/1999/03/31/FD91867.DTL
As mentioned, I'm not aware of any shortcomings with the blend. What is everyone else's opinion/experience?
I buy the Wal Mart brand of pow sugar and it's beet sugar. The only other sugar I've ever used is C&H and I see no difference, except in the price. I've looked for the 10x sugar ant not found it around my area. I was going to give it a try just to see if there IS any difference. I sift all my pow sugar before using it. Just thought I'd get my 2 cents in since this has come up again.
I have only found two domestic (oops - make that US since we are an international group) companies that advertise "pure cane" sugar - C&H and Domino. I have not found C&H in this area, but Domino is readily available at a higher cost than other brands. I have tried Domino's beside Costco's and cannot tell any difference in taste or texture of frosting 
. Wilton has nothing to gain by advising use of pure cane (at least I don't think they do ), so there must be some evidence of superiority of some sort. Or maybe Wilton does own part of C&H or Domino 
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