I am planning to use radiant heat in the slab on my new two story in Arizona.  I would like to grout large (20") tile on the floor.  In a conventional slab, I would be installing post tension cables to prevent cracks.  Would this compromise the pex tubing?  I could go with 6 bag and fiber concrete and hope for the best.

I suggest you go over to the John Bridge forum and research the tons of discussion about concrete slabs and cracks. I think what you will find is make d*** sure the base is solid, I mean solid, use the fiber reinforced cement and save the cost of post tension, and cover the slab with Schluter Ditra uncoupling membrane under the tile. That way the concrete can crack, which it WILL do, without the cracks affecting the tile. My concrete man said you will have to use wire mesh if no other reason than to keep the PEX tubing from floating up out of the wet concrete.