I think the problem is that because with some canon cameras, those "tweener" iso's are fake (in other words, ISO 125 may be the same to the hardware as 100, then hacked by software to look like 125) you might get more quantization error and lose a little dyamic range (either as lost highlight headroom or increased shadow noise, depending on which way the software adjusts).


Personally, I don't care about any of that. I just turn them off
because I don't like having to turn the dial so much to go from 100 to
3200.


You always get less photon noise when using a lower iso (assuming the meter reads the same, the light actually is the same, and you don't adjust the exposure) because you get more light, so I don't think the claim that ISO 400 has less noise than 250 is always true (read noise may be worse, but overall noise will not always be... in fact, will probably not be in most cases).


I think Daniel can expand/correct the above.