Currently, you can work with 16bit float OpenEXR images in A:M. TGA and PNG images are 8bit, so using OpenEXR images for color or bump/displacement gives you more precision (which I've really noticed in displacement maps), but, depending on the size of image you are rendering, you may not notice that difference. However, using OpenEXR images for maps makes them 1.0 gamma automatically, so there isn't a need to color correct the maps when working in a linear workflow. I also render to OpenEXR so that the linear workflow is maintained. It makes post-processing essentially lossless when you have a final image that is 8bit.
Hope that helps, Dan.