Oh boy, Cereal can be quite difficult for a diabetic. To answer your question, you should be paying close attention to the number of carbs, in total, in your cereal. Cereals like bran have a higher fiber content, which may make your "net carbs" lower, because the fiber makes the carbs take longer to break down. I have had type 1 diabetes since age 14, and in general I try to stay away from cereal. Its perfectly fine in small servings, and of course without the added sugar, but its packed with complex carbs which almost always cause a high BGL, at least for me. Remember that rasberries and blackberries are two of the lowest sugar fruits, which are great in bran flakes with skim milk!
I'm the same as Live; I can't eat cereal at all, and if I add fruit as I like it, it spikes me horribly. I eat eggs for breakfast. But if you are asking if you subtract the fiber carbs, yes, if it is more than 5 you can subtract them. But my experience is even if I bolus for the carbs listed I still go high.