I have never made icing with shortening, only butter. I usually make either swiss buttercream or an egg yolk enforced buttercream.
Do you use shortening and then add artificial butter flavor for cost reasons, or does the shortening have properties that make better icing ?
I use all shortening buttercream and add butter flavor.
One of the good properties of using shortening is that it does not need to be refrigerated. It also crusts nicely.
Since we're on the subject of frostings, I have a question Y'all might be able to answer:
Do any of the cooked (flour with milk and boiled) icings, or whipped icings crust over like the shortening buttercreams do? Can you pipe with those icings as well?
I've only used the buttercreams to decorate with, and I've used a couple of the whipped as fillings, but that's it.
--Knox--
The first and only cake decorating class I took taught us to use powdered sugar, crisco, flavoring and water. It is the only icing I have ever used for making flowers and other decorations. That was the only class until recently and I am taking a gum paste class. I have learned so many great things since joining this site, can't wait to try buttercream. 
I use shortening because it makes a very white icing AND it keeps a very very long time and I don't have to worry if it isn't refrigerated. (I don't use milk either for the same last two reasons.)
I'm also a shortening person .... and not just any shortening ... Crisco only! (I told my Sysco sales person that I would not be using their shortening again ..... it not only didnt' meet our standards for the icing, but left an aftertaste in my cookies!) Butter has a much lower melting point than shortening, so I'm shortening only and I've never had icing melt-down in over 20 years. The only flavoring I use is pure clear vanilla. (unless the bride wants ivory icing, then I use the cheap brown stuff to get the color right). 
I use a mix, depending on the event. If I have to make it the day before, or if it's too big to fit in my fridge, I use all crisco. I prefer the buttercream dream recipe on this site, it seems to work the best for me.
I use shortening for the stability factor. I also use a part shortening part butter recipe. I used to use an all shortening recipe but found it much too sweet, so I use this recipe now: http://www.cakecentral.com/cake_recipe-1602-ButterCream-Icing-for-Frozen-Transfers.html
It gets rave reviews all the time!
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