Depends on whether you want one really. DSB
in a fuge is a lot more common. If you have sand sifters or jawfish you're going to want a DSB
of at least 4"
It is definitely beneficial regardless of whether you have a sump. You can get away with a somewhat deep sand bed without appearing to be a deep sand bed.
At the front of the tank start off from 1.5-2" and work your way up to 4 inches at the back end. This helps detritus collect from the front and gives a ton of room for sandsifters and beneficial bacteria.
But in all honestly a DSB
isn't necessary. Try to have at least a 1.5-2" sand bed and rest any base rock on the bottom of the tank before you add the sand for stable foundations.
In my 24g I have 2" in the front and 4 in the back, but it doesn't appear deep at all.
Just make sure you have something to aerate it every once in a while like nassarius snails, bristle stars (though not until tank is mature and even then...meh) or a sand sifting goby.
The sand bed doesn't need to be pure live sand by the way. You could mix 10-20 pounds of live sand with aragonite and have pretty much the same result. Just for the love of the oceans do not use beach sand or playbox sand O_o