Animators Bill Plympton and Patrick Smith have a joint blog where the discuss animation issues. Recently Smith posted about the Rule of Thirds and showed some sample images of it at work in movies.
Here's a Rule-of-Thirds grid you can use in A:M
Click to view attachment
Import it to your Images Folder, then drag it into your camera view. Select "Rotoscope" on the menu that comes up and then it will automatically size itself.
Compose your scenes with the white lines as a guide and then turn the rotoscope off before you render.