If you are doing sencils on a buttercream cake, let your iced cake set for at least overnight. The will help the icing crust up and it will be a lot easier to do stencils on. Also, using canned icing for the stenciling works real well. The icing is a nice soft consistency. Gently stir the icing a little before you use it. Of course any time your stenciling on top of buttercream, you will need to be a lot more gentle than on a fondant cake. But it definately can be done, because I do it all the time. And of course every time you use the stencil, you will definately want to wash it off in very hot tap water and dry it thoroughly. Otherwise you will run the risk of getting icing on the cake where it shouldn't be.
I know, it's a pain to have to clean it every time, but you'll be glad you did. It also helps to practice a bit beforehand on a flat surface, just to get used to the technique. Always spread the icing in one direction, and only use a very thin layer. Good luck!!!