The consensus seems to be that, unless you have something to bake right away in which you could use the buttercream, you should freeze it. That's right: don't toss that buttercream! It will remain good in the refrigerator for 4-5 days. Alternatively, put it in an airtight container and freeze it up to 3 months. When ready to use again, take out of the freezer and place in the refrigerator overnight to thaw. Before you frost the cake, let the buttercream come to room temperature and give it a good stir before piling it on.
When I was talking with my aunt the other day on the phone, I mentioned this and she said she always keeps a container of buttercream in the freezer in case she needs it. While I can't say I'd ever go quite that far in my quest for cake-preparedness, I am happy to learn that buttercream will never be wasted again in our house.