They tell you don't place it near the scribe line because stress at the edge of the chip is maximum.
Since thermal expansion of the IC is different than the leadframe, you can consider the chip bending very slightly. The exact center does not bend at all because the forces on either side are equal. The four sides bend more. So it places unequal stress on the sides of your your delta-Vbe and you get offset.
But really, as long as you don't place it in the corner it's fine. At the top scribe, dead center has worked for me many times. But I trim my reference so small random offsets don't really matter to me.