Regardless of what modern literature says about the matter, I find no reason to make up large batches of synthetic seawater and store it.
I find it hard to concieve that a hobbyist with an established tank that regularily does water changes and water tests will run into a situation where more than 20% of the tank water would ever have to be changed at once (one helluva run-on sentence, eh?).
The longer your saltwater sits, the more of it evaporates. This just waste distilled/ro water in my opinion.
BTW, I'v always mixed my saltwater right before I add it to the tank with no ill effects on fish, corals, or other inverts. I do 5% changes every week, and that is one benefit of doing frequent, small changes.