Is it okay to leave buttercream made with milk in an air tight container out for a week before I use it?
It's okay to leave out for a day or two, but I don't know if I'd leave it out for a week. You can put it in the fridge or freezer, then just remix before you use it.
As I was taught, if is made with milk, it has to be refridgerated I might be wrong, but my Wilton Instructor told me so but let's wait to see what the other CCers have to say the more experienced Decorators.
I use a modified Wilton recipe for buttercream. Their's has milk in it and on the icing information they provide on their website, they indicate that an iced cake can be stored at room temperature for 2-3 days.