Has anyone ever done this? Baked cupcakes and used them for practicing cake decorating techniques. Like, baked them, unwrapped one and stored the rest, and decorated the one cupcake as if it were a regular cake. I'm thinking of doing this so I don't waste too many of my materials (fondant, buttercream, cake mix).

No.

No.  Get some cake dummys.

www. dallas foam .com

Another option is to decorate a cake pan that is turned upside down.

AYou can also keep your buttercream in the fridge and reuse it over and over again on dummy cake. Just put a label "do not eat" on it, just in case! You dont want that stuff on a real cake by mistake...

Cake techniques don't translate to cupcakes very well. While practicing on dummies or cake pans are fine for specifics, like piping, borders, flowers, it's my opinion that you need to practice on real cakes. Dummies and cake pans don't get bulges, don't bake unevenly, they don't shift or settle, and stacking them doesn't really count since there's no way to learn how to properly support a cake. 

If you use a cake mix, and make a practice buttercream, it's not that expensive. Splurge. Practice on real cakes. We've all seen the websites with beautiful dummies next to lumpy, bulging, misshapen cakes.