When I started "animating," I would film a shot with my old video camera, pause the live footage (a feature I discovered on that camera) and then play the next frame... With this process, I would speed up the footage in the computer so that it appeared as if I had done true stop-motion. (Which, it was, but, in the form of video)
Recently, I looked back at those films, and clocked it at approximately 8 fps.
After then, I got a still-camera and would animate at what I thought to be 15, but was actually 12 fps.
Since early 2011, I have used "true" 15 fps. However, I always have been looking to improve the look of my films... And, taking inspiration from the "look" of the films of One Brick Studios, MindGame, and Starflight Studios, I have tried to film on 2's at 24, AND digitally adding an "onionskin blur" frame between all frames: i.e. f1, f1+2, f2. In the end, I've found that the extra time spent editing in these extra frames isn't worth the oh so just barely smoother animation! At least for me.
So, I stick with 15 fps... However, for a test, or even if you're just lazy, it's fine to animate at 12 fps...
However, if you DO choose to animate at 12, I would suggest testing out the "onionskin blur" frame thing, as it makes 12 look more like 15. 
(Onionskin blur frames can be achieved easily, in GIMP, by setting the "above" layer to 50% opacity: and saving that frame as a between “1” and “2” frame - like “1+2.png”)