Jason's right, stick with one salt. If you find you don't liek your salt, change slowly, using more frequent, but smaller water changes.
Every salt brand has its own unique mix. Doing a change with a new brand will affect inverts and corals the most...your fish won't mind as long as salinity stays constant and it's not radically different in something like alkalinity, pH or other traces.
That said, I'm not a fan of IO salt. I started with it, and like many people, mine would never fully dissolve, and I found it had really poor calcium levels, with a somewhat high alkalinity (compared to calcium...they weren't balanced).
I switched to Oceanic brand, and I really like it. I think its a better salt if you plan to have corals, or want decent coralline growth (it has more calcium, and less alkalinity...they are balanced...and it dissolves very easily)
Former advisor and planted tank geek...life's moved on though.