No, the runtime can't force shaders not to draw outside the object's bounding box, but behavior if you do so is undefined. Pixels outside of the bounding box will be whatever is left over from previous rendering operations that need an intermediate stage. Also, if there is a second effect in the chain, the next shader only processes the bounding box, and the bounding box is the only area copied to the display, so in some cases it will be truncated anyway.
Basically, although you can, you should not write shaders that access outside the bounding box.