"Yes they are rectified, when I had them laid out looking at them at the store they seemed pretty damn flat, I realize that's not scientific, should I put a straightedge on them?"
Rectified is good. With rectified, large sheets are fired and the tile is cut to size after it is fired. It results in a more uniform sized tile with less warpage. Generally, flat tiles. Still, take two tiles and set one on top of the other, glaze-to-glaze touching. I understand they're probably through-body porcelain, but you know what I mean.
See if they rock against one another. Do they touch in the middle and are they gapped at the ends? 48" tiles often have a bow to them, and that's why others were mentioning the offset limitations. Often times the manufacturer has an offset limitation in the material literature. Typically 25% to 33%.
Using a 50% offset (bad idea) with a large format that has a bit of bow to it could result in the lippage the others were talking about. Toe stubbers.
Even if the slab is in terrific shape, I'd recommend something like NobelSeal CIS between the slab and the tile.
Hope it turns out well!