I had this exact same problem yesterday and I used just a pinch of popcorn salt. Use popcorn salt because it is a much finer grain than regular salt and won't leave little spots in you icing. Don't put in too much though, just start a little at a time. Hope this helps!
I had just written on this subject yesterday....I made the best, smoothest, not-overly sweet buttercream I ever made because I used sweet-cream butter instead of unsalted butter (by accident). The butter (Kirkland brand from Costco) is slightly salty....not as salty as "salted" butter, I thought. It totally cut the sweetness and I will use only that from now on.
I know that the salt idea works, but sometimes I also add some liqueur, like Kahlua to chocolate buttercream, or Baily's etc... I made a cake yesterday that I added almond extract to, but I really wanted to use Frangelico, which is hazelnut, but one of the guests is allergic to nuts, and I was unsure if it would affect her...
I read a post the other day about this. Someone here...I can't remember who...makes a batch of regular buttercream and one of IMBC, then mixes them together. The non-sweetness of the IMBC cuts down the sweetness of the regular BC.
Personally, I'm dying to try this one out 
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