I don't have up-to-date figures of smartphone power consumption, but I think that CPU and GPU should be somewhere at the same level. Correct me, if I am wrong.
Generally, trying to optimize parts of your code that rely on performance is always a good thing, not only because you save battery life, but also because you can reach devices with less powerful hardware. I also always encourage people to use OpenGL ES 2.0 / OpenGL 3.0 for games, because the power of shaders is, like you said, to cut down the effort to a minimum and to provide much more flexibility.
But, based on the assumption you are rendering some 2D sprites, I'd say that the GPU is mostly in an idle state. That means that the CPU will possibly have more impact on battery life than the GPU, so you should not expect too positive results here. I'd recommend you try to improve your algorithms running on the CPU before you optimize the ones on the GPU, but I'd use shaders too for the sake of flexibility. At least if you can ignore the devices out there without support for OpenGL ES 2.0 (again, I don't have figures ^^).