I wouldn't put sand in them but some liverock and macro or xenia is a good idea if your getting enough light to them. Without sand you could just hook a hose to a powerhead and flush them out once in awhile if they build up detritus.

One of my overflows is full of thousands of mysid shrimp, pods, and a bunch of zebra hermit crabs. I have know idea how the hermits managed to get in there. In the other overflow I keep firefish skeletons :( .