Ok, here goes the chemistry.

If you add a tiny amount of iodide to a developer which uses a low level or no iodide, then the iodide goes onto the grains instantly, but then is released imagewise proportional to development. It therefore concentrates in areas of high development and causes edge effects.

With a high iodide emulsion, the iodide is already there in the emulsion ready to be released imagewise to do the same thing, but additional iodide acts to supresss this effect, kind of like buffering the iodide ready to be released from the grain. Also, todays emulsions vary in iodide content, and so it is hard to choose the level of iodide appropriate for use in a developer and so results (accentuation or supression) might vary from film to film. In addition, it is important to remember that some modern emulsions are core shell with iodide in the core but not on the surface. These react in a totally different manner.

So, there is no one answer except to say that adding iodide is a bad idea as a general rule. It may work, it may not work at all, or it may work counter to expectations. It is therefore considered best left out with modern emulsions.

That is why I have said before that older developer formulas may not be best for modern emulsions and vice versa don't use modern developers on old style emulsions.

Does this help?

PE