I just find that its the best for me, Ive done a lot of skinning of characters and I found I had to do a lot of work skinning the shoulders in a T to stop pinching, having the 45 degrees down made things easier and gave nice deformations. Also having the palms down I found got rid of problems sorting out the twist in the arm via the joints, for a simple bit of modeling.

I cant see any advantage beyond maya, Z just sculpts, if its motion builder or endorphin then dont they generally have palms down (even though they dont really care)?

As you say its up to the person.