Natural saltwater is around 1.025. It does vary in different areas. So in the interest of your fish I would keep it close to there. Never heard any expert say 1.020 to 1.022 will make your fish breathe better.
It's a fact Mike. Check it out. If you have fish only, your fish actually like a little lower salinity. I'm no expert but it has something to do with the way they get their oxygen out of the water. Different from freshwater fish.