Can you use Regular egg whites or powdered egg whites in icing? Which is better? Would it affect the way the icing turns out, is this like a shortcut to not having to heat up the egg whites to where they're edible?

If I'm making an Italian meringue buttercream and I know people with immune systems that are potentially susceptible to food borne illnesses (e.g. kids, pregnant women, elderly) will be consuming it, I'll use the Wilton Meringue powder instead of fresh egg whites in my normal recipe. It works great. There is a slight taste difference (due to the extra stuff inside the meringue powder), but it's great peace of mind.