Polished porcelain tiles and screed

I reckon its pretty optimistic to think you could lay a 10mm screed throughout a number of different areas/rooms more evenly than the concrete slab which was poured and levelled without any walls getting in the way....

If it was a bit out of level over the length/width the tile adhesive makes for a bit of forgiving ...Unless it was obviously bumpy I would not worry about it. With tiles I reckon it is not so much how much out of level they are over the entire job, but how quickly the levels change.

If you had a reasonably consistant gradual fall of 10mm over the job that's not necessarily going to be a problem. When the fall is inconsistent with bumps and rises in a relatively short space that is noticeable - than that would be a problem.

If you HAD to do something it would be easier to put down tile underlay to even out some of the bumps.