For posting on web or printing out at 4x6 size, I would not hesi
For posting on web or printing out at 4x6 size, I would not hesitate to use ISO 6400 when needed on 1D3 (D3 should not be worse). Noise at ISO 6400 is barely noticeable. So I would guess ISO 12800 should be very useable for posting on web or printing in newspaper.
