I came across this video on YouTube yesterday, about setting up customizable buttons on the 5DIV. I don't think he gives the best advice on the actual set up, but he shows possibilities I was unaware of - that you can make detail customization to the chosen AF mode. It works the same way in the 5Ds/r. To me, the button customization possibilities he demonstrates are fantastic.
https://youtu.be/D_R77Gs8Q6E
https://youtu.be/D_R77Gs8Q6E