When using fondant accents (on a buttercream cake) do you cut them out and let them dry or place them straight onto your cake?

I let then set for about 10 minutes so they don't lose their shape while handling.

I have done it both ways, but if they are to go on the sides of a cake, I would let them harden a little like Moria76 suggested.
If they are going on the top, such as a sheet cake, if they are not too flimsy, you can pick them up and put them right on the cake.

Happy "accenting".