I've never had to piece a back. I haven't made that many that would require it. On the ones that did, I used 108". If I had to I think I'd go with the diagonal seam. Makes the most sence to me.
As an afterthought, If I had to piece a back I would go with leftover blocks for more interest. Not centered, of course. I suppose there are many different way one could do it depending on that individuals thoughts. To each their own I guess.