Well, my thinking is that since solubility is dependent on temperature (among other things), when the temperature drops things become less soluble and come out of solution - and crystallize. In my thinking, this does not mean that they have oxidized necessarily. Similarly, shaking a solution creates some warmth and so increases the temperature, making it easier for things to go into solution.
I may be all wrong on this - if someone else knows better, please say so. But it seems to be a matter of solubility and not oxidation to me. I have had no problem yet with the pH of the vitamin C/distilled water solutions that I have been using as a spritzer. No discoloration. So this says to me that they have not oxidized and so are safe to use. This is just my opinion.