The wonderful gal who introduced me to decorating cookies said someone taught her that if you don't have bowls, spatulas, etc., you use exclusively for icing, then you must, before each use, wipe each item with vinegar, then rinse and dry, or oil residue from other baking uses will interfere with your icing. This is a bit of a hassle, so I thought I'd check with others on this. Do you wipe with vinegar? Thank you!
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No, I've never done this in all of my years as a home cook or professional bakery owner and never in the making of royal icing - and never have had any issues.
It's true, a tiny bit of grease will interfere with the formation of a fluffy, voluminous meringue, so when volume is paramount to an egg foam/recipe (i.e., for Pavlova, mousse, Italian buttercream to a lesser extent, souffle, other meringue-centric recipes, etc.), then I make sure my bowls and tools are completely clean by washing them well with dish soap and water, that is all.
But with royal icing, the ratio of powdered sugar to egg white is so high that any egg foam you might create is immediately deflated by the sugar anyway. Plus, I'm not in the habit of over-beating my royal icing, because it introduces air bubbles into the end product. So maintaining the volume of the egg foam in it isn't altogether that important in the scheme of things anyway.
Oh thank you! Your reply makes me think of another question, if I may. I know it's important not to overbeat the icing, but sometimes it just does not get stiff in the normal 9 minutes or so. How long do you generally let it go before you decide to add more powdered sugar? Thank you!
Sometimes I wipe my KA bowl with vinegar but only when making meringue. As Julia said, you're not looking for volume in your RI or corn syrup glaze (that's the one I use).
Cindy, My royal icing recipe mixes to a thick "glue"-like consistency to start (then I thin it for different tasks). It gets very thick with only 1 to 2 minutes of beating, unless the eggs are especially large large-grade eggs. I whip it just until it thickens and turns very sharp white. I've never in my life beaten royal icing for 9-10 minutes as some people do. I don't have the need or time for that! 
But every recipe is different, and people should use what works for them, first and foremost. My royal icing recipe can be found here: https://www.juliausher.com/kitc...o/recipe/royal_icing
