If your surface area of the dsb in your sump is comparable to the surface area of your main tank then by all means go ahead. A DSB in a sump can actually be more efficient than a DSB in your main tank as there will be less obstructions on the surface of the DSB.
A DSB is not a must. Many german tanks do great without any DSB. However, you will have to find another way to prevent nitrate buildup. Can be accomplished via excessive skimming, low bioload, less feedings, 24 Hr photoperiod in refugium, weekly waterchanges etc.
I myself using DSB in the sump as I fear my main tank cannot take the load of so much sand (ugly to look at too). The surface area of the DSB is only about 1/5th of the surface area of the main tank. Somehow nitrate levels have dropped by almost half since I've set up the DSB in sump. Amazing.. Might be due to my macroalgae, but I doubt it.. What works for me might not apply for others..maybe fishload,etc are different? If you decide to go for a DSB in the sump it would be best that the surface area of the dsb be as great as possible (Again depends on the space you have below your tank).