Given a laptop with a dual integrated/discrete GPU configuration, is it ever more power efficient to use the discrete GPU instead of the integrated?

Obviously when writing an email or working on a spreadsheet, the integrated GPU will always use less power. But let's say you're doing something graphics-medium but not graphics-intensive/heavy - is there a point where it actually makes sense to fire up the discrete GPU, not for performance but for power-saving reasons?

Off the top of my head, I can think of a scenario where the external GPU supports hardware decoding of a particular video codec - I'd imagine there is a "price point" where using the GPU saves more energy than decoding that fully in software would. But I think most GPUs, integrated or discrete, pretty much decode just the plain-Jane h264.

But maybe there is something more complicated, perhaps if you're doing something like desktop/windowing animations or a flash animation on a website (not an embedded flash video) - maybe the discrete GPU will use enough less power to make up for switching to it?

I guess this question can be summed up as to whether or not you can say beyond doubt that if you don't care for performance on a laptop with two GPUs, always use the integrated GPU for maximum battery life.