craftgrrl | Stencil sheets as shrinky dinks?

I've been searching the memories and working my Google-fu, but I can't seem to find an answer to this question. I've been looking into shrinky dinks as a way of expanding my jewelry making, but no place in my town sells them (well, there's a Klutz book at my local bookstore, but I don't really want to pay $20 when I can order shrinky dinks sheets online for much cheaper). I don't have any #6 plastic around right now, but my mother swears I can use some blank stencil sheets that I have lying around as shrinky dinks. I, however, am not so sure, and short of putting some in the oven and seeing what happens, I have no way of knowing if this will work.

Has anyone attempted using these before? Should I go ahead and try it, keeping a can of oven cleaner and a fire extinguisher on hand in case Very Bad Things happen? Thanks for your help!

Edit: As it turns out, Mylar stencil sheets are a #1 (PET or PETE) plastic, which won't work for shrinky dinks. Thanks to everyone who helped!