Actually, my habitual downrating of color neg started about the time when we converted from ASA to ISO. It was a time when many films were off by about 1/10th of a stop to 1/3 of a stop. Fuji color neg was a full 1/2 stop slow IIRC and to boot, the 3 layers were not balanced for daylight. It was 1/3 stop slow if the color balance was adjusted in printing.
In any event, I learned to use that 1/3 stop to advantage and have kept on doing it in my non-DX cameras. In my DX cameras, I let the camera set the ISO rating. I recently exposed some Portra 800 at 400 by accident and the results were just fine. I had left an old tab on the camera back for Portra 400. My bad! I had forgotten what I put into the camera due to old age.