from the standard reefing texts and what maintenance guys have told me you only need to make sure the salt is dissolved then its good to use. I think they were referring to IO because its a simple salt, one of the quickest to dissolve and consistent (also the cheapest). they would start mixing right away when they got to a job and used it the same hour. IME it depends a lot on the salt brand, and the more "complex" the salt mix the more considerations such as:
1. the salt must be completely dissolved. there must not be any more particulates. salt that is still dissolving is considered caustic if I recall.
2. the additives must be dissolved or have completed their reactions. this is where reef salts can be confusing because the typical addicitives like Ca HCO3, Mg and trace sometimes take a while, and presumably undergo a series of of reactions then re precipitate out (that brown crap). so its hard to tell when you get the desired final product
3. some additives degrade and must be used immediately, like the vitamins in Reef Crystals or the organics in TMP / Bioactif
4. NSW has organics and living things and must be circulated and aerated like the DT tank water. I try not to make my mixing station biologically active for this reason.
Ive made 200g batches of salt water (TM, TMP, TM bioactif, IO, RC, Salinity, Oceanic, etc) and kept them circulating for a month without any change in chemistry I could dectect. I used to aerate to help match pH (increase pH most of the time by blowing off CO2), and even had a chiller attached to one of my previous mixing stations to match temp but that was a lot of work.
