there are opinions on all sides of the fence on this subject but basically IMO they should be one of three options, Bare bottom (no sand) under 1 inch or over 5 inches. a lot of people have successes with many different set ups and I am definatly not going to start the DSB versus SSb debate, its been over done a million times. I prefer DSB's my 65g has a DSB of 6 inches, my 75g has a SSB in the DT with a 5 inch DSB sump. I think the de-nitrification is worth the extra cost in sand, as well as the volume in tank lost due to the sand bed.
but you need to take into consideration tghe critters you want to house, if your going to house fish that constantly shift sand a DSB may not be a good choice as they can damage anaerobic zones causing constant fluctuations in your tank by moving sand around.