I have been looking for an answer to this but no-one seems to be able to provide one.

I have heard it affects the tanning chemicals but have also been told that this is untrue...

Others say that the iodine tints the leather - which is not an issue with most hair-on mounts.

I was also told that 'iodized is unsuitable' advice it is more 'urban-legend' that has been perpetuated as fact.

So, whats the opinion of those here?

Seems lots of guys use iodized table salt to treat hides with no ill effects.

Been using regular iodised salt for my own hides/skins since i can remember with no ill effects.
The only salt i really dont like people using is rock/pool salt.

Just an old wives tale.
In the late 19th and early 20th century, the hygiene factor along with the diet led to an incidence of goiter in humans. To combat this, iodine was used in minute amounts. Though deadly poison in bulk, it could be administerd in minute quantities. The government decided that one of the most inconspicuous and long term healthwise was was to include trace amounts in table salt.

Tanneries used volumes of salt and knew that iodine would stain and knew that if it contacted leather or white hair, that too would stain. Not bothering to consider that such amounts were infinitesimally small and would never effect the tanning, the rumor started and is still repeated in many circles. Ain't so. Salt is salt. Just don't use the "high mineral" cattle salt or salt the has a sulfur additive that tints it yellow. Rock salt is also out as it's too big to be effective. The finer the salt, the better. Table salt or baking salt are exceptional choices.

Iodine is not added to table salt as "iodine"...it is added as potassium iodide, which is colorless and highly water-soluble

Conifer, I'm assuming you mean pure iodine is not added to salt. That's correct but by compounding it into another element doesn't mean iodine isn't put in your salt using a carrier compound.

Perhaps this says it more clearly: Edible salt can be iodised by spraying it with a potassium iodate solution. Two ounces of potassium iodate, costing about $1.15, are required to iodise a ton of salt. Salt is an effective vehicle for distributing iodine to the public because it does not spoil and is consumed by everyone in the population, in fairly predictable amounts.

The main reason taxidermists use non iodised salt is cost. 50# of non iodised salt cost less than $5.00. The big thing in recommending salt use to hunters is for them to buy the finest grained salt they can such as mixing or feed salt. This is because it penatrates and soaks up juices and dries out the hide and set the hair roots fast which is what you want. Rock salt tends to just make a mess and thats about it.

What about Borax? I have used it....it definitely precludes bugs, and helps to dry the hide. Is it useful in a general sense for trophy preservation?

If you plan on having the hide tanned, borax is not your friend and will spike the pH of your acid. Many people confuse borax with dry preservative (DP). Borax is more of an insect inhibitor that a drying agent. With DP, you have a lot of unscented talcum added that acts as a drying agent and alum which acts as an astringent to the cell structure. Once a hide is DP'd, salting is not as effecient.