It's been a few years since I took cake decorating courses, but when I took Wilton 2 and 3, my instructor gave us an alternate recipean alternate recipe to use. It might not crust up quite as fast as the one with meringue powder, but it was fine for the humidity that we have in Maryland. (although, I admit, DC metro, not eastern shore). I've made it a few times using meringue powder in place of the cream of tartar ... I assumed it had to be a mistake the first time I made it, and both work fine. I haven't done a side by side comparison to see which one crusts up first, but I'm guessing the meringue one.
To respond to some of the comments made so far ... Wilton Buttercream is not a real buttercream ... it is stable at warm temperatures, and is much less finicky when trying to make flowers, etc. It's also pure white, so you won't get off colors when you are trying to tint it. I'd leave out the butter flavor, as you don't need it ... The sugar and fat pretty much hide any flavors you have.
And one other possible source of off flavors -- oxidized shortening. I keep a can that I use for greasing pans if I'm not specifically using cake release, but for frosting, I use the sticks. They're tightly wrapped, so less exposure to air, and easier to measure.
