I've always done a little sand, then all my rock, then more sand. I've always run a DSB, as in 4-6" worth with no problems. I've never had issues with sand creatures digging down all the way to the bottom, or even a little ways and causing the rocks to shift. The first layer of sand was to prevent the rocks in some ways from cracking the bottom pane of glass, and then the second and final layer just filled everything up and looks more natural to me.
Good luck with whatever you decide!
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