It really depends on what you're doing in your QT tank. If you've decided that your'e going to QT everything under the hypo salinity protocol, then you keep your salinity exactly within that range. If you're not doing hypo, I can't really see a reason why you would hold the salinity at anything other than what you keep your display tank at. Keeping it at slightly lower salinity but not low enough to call it hypo could potentially mask certain problems and erroneously lead you to believe your fish are free of parasites when they are not. I know of at least one LFS here that does that, every time I bring fish home from them I test the SG in the bag water at 1.018. It's not low enough to outright kill something like ich, but it is low enough to suppress it to the point where you might not notice it.
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