Ok, so I think I have a tolerable understanding of the concept of increasing the ionic strength of aq. To force the freebase molecules to the nps. And that it is also the reason that filtered water is ok to use because it has higher ionic strength than non-ionized h2o.
My question is actually regarding the acid soak phase. It would stand to reason, that it would be preferable to use non-ionized h2o with nothing already dissolved in it during acid soak so that it could hold more of the water soluble acetic molecules. Would this be correct?

Not necessarily. At lower ionic strength there could be salting in effects. Also, since acidic water has its own ions (H+ and A- themselves), the contribution from tap water to the total ionic strength would be second order/negligible.

The only place it may matter (in my opinion) is for cell swelling before freezing. However, I've tested tap water for this and got great results.

I have not noticed a difference anywhere and simply use regular water.