I think it depends on what grains you are steeping. Some, like crystal malt, mainly add color and don't contribute any fermentable sugars. That is my understanding, anyway. Roasted grains are said to be the same. The grains selected for steeping are usually picked because of other attributes than fermentable sugars, like flavor and color.
But, I will defer to the more knowledgeble on this question. I seem to remember another recent question like this, but I can't find it.