since the OP mentioned for SPS tanks, i really think the salt choice might even be less important due to all the consumption of the corals on nutrients. Either way you are going to have to dose, so if the levels are a little lower i dont see the issue. For example, if you are doing a 10% water change, and your existing mag level is 1450, and the new water is low at 1300, the resulting level in the tank after you change water only goes down to 1435 (from 1450). that's not enough to worry about, and for me not enough to spend more money for a salt with slightly higher levels.
Obviously the difference is even less for lower level items like calcium, etc