From what I have read, some keep them with no problems. Others report that they stir up the sand layer too much and therefore interfere with colonization of nitrate-consuming bacteria. The total depth of the sand layer probably plays into it too, as the deeper it is, the more it can tolerate disturbances at the top without affecting the nitrate consumption. Last I heard, for a good DSB, the sand layer should be 4 inches deep.
sam