The manager at my LFS, said he didn't like bare bottoms ,even in QT tanks, because he's seen fish be stressed out by them. Fish don't understand the reflections they're seeing on the bottom of the tank after all.
Since his advice of a minimal substrate to break up the mirror effect runs counter to the advice I see on here I decided to split the difference and line the bottom of my QT tank with Indian almond leaves.
I only made this change a few days ago, and have no idea if I'll keep it, but in the short term I'm happy with it.