Oh definitely use unsalted butter when making meringue based icings. If you use salted it will taste like sweet butter... good on scones, but not so much on a cake. It tastes more like vanilla ice cream when you use unsalted butter. I wouldn't use shortening in a meringue based icing personally as you really will taste it since there is so much less sugar in these recipes as compared to a powdered sugar based icing. The more vanilla you add the better it will be.
I don't like powdered sugar based icings at all, but using 1/2 salted and 1/2 unsalted is better than all salted. I don't love shortening, but 1/2 and 1/2 in powdered sugar based icing isn't bad at all.
