A crystal (something with "cara-") can be steeped since its already had the starch turned to sugar. A highly roasted grain can be steeped since its mostly there to add color and flavor, its starch has been burnt up to a great degree. Any malt that has its starch intact (pils, 2-row, pale, Vienna, munich) needs to be mashed to break down the starch to sugar. You could get some flavor out of it but the starch is liable to cause cloudiness.