Well here is my 2 cents.... I have been told that if you use milk you can let it set out for a day or two. When made with sugar it is a perservative and won't spoil in temps under 80 degrees. Have done this my self and the icing was fine. Butter icing I left it out for one day till I delivered it. Did not have the room in my fridge for a 14x10x8 stacked cake to stay cool. My house was very cool. It had snowed the day before so no problem. HTH
I only ever use all butter buttercream and have left it out overnight (until delivery) on nearly every occasion. It will be fine - your only problem would be in the summer or hot weather months - then it starts to melt!
If you can leave butter by itself out in your location, the buttercream will be fine and won't melt. If you must refrigerate your butter, it would be best to do the same with the buttercream. If your question was 'will it spoil', the same answer applies.
Sarah Phillips from baking911 has a discussion on this. It's very useful.
http://www.baking911.com/asksarahbb/index.php?showtopic=1299
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