When first getting into this hobby, I talked to a friend at the Florida Aquarium in Tampa(FL), and asked them what they used for their smaller tanks. They said they use a blend most similar to Tropic Marin, but a commercial type. Said it was one of the more scientifically refined. They said that all the salts were pretty much equal for the 'non-professional' use. They said that Instant Ocean/Red Sea works fine, just that the strains of trace elements are not as pure as that of some more expensive salts. Said that most of the best can be bought at scientific research supply warehouses. Ofcourse, in there huge tank, they have a barge fill up with water from the middle of the gulf and bring it back every 3-6 months for their water changes. I'm pretty sure that tank is over a million gallons, but i can't recall...
Seems that professionals (being Marine Biologists) don't think it is very crucial for home use as long as you don't use extremely cheap salts.