I scoured the web when I was trying to figure out what depth I wanted my sand bed. To summarize... seemed like everything I read contradicted the other. Even within wetwebmedia, I found opinions that conflicted. Granted... there are always different opinions on things, but you can usually get a feel for what's valid and what's not. But DSBs... wow.
Anyway, I ended up with a hair over 3". Towards the front it's maybe a hair under, in the back probably closer to 4". After my cycle, I had maybe 5ppm nitrates in there. I did several big water changes, but could still detect a trace of nitrates. I then started doing my 10% weekly PWCs which I've continued to do for 6 months now. I have no detectable nitrates. What was there was either diluted enough that I can't read it, or was eliminated.
I've only got 1 fish in there so far, feed once daily with minimal leftovers, and have a beefy skimmer for the size of tank (Remora Pro in a 46g), so my nitrate levels shouldn't be bad to start with. I run GAC
24/7. I have no idea if my sand bed is contributing to my lack of nitrates. Probably not, but I have no way of telling. After a year or so, I might have a better opinion but it will be just that, an opinion.
So I guess I'll just weigh in with a "I've kinda got a DSB
and it *could* be working" opinion. In the end, I'd say go with what looks best for your setup. For me, I put the rock work directly on the tank bottom and I filled up to where it looked "natural." Since then, I've rearranged some sand to expose a few more "caves" but for the most part have left the sand bed alone.