Hmmmm. Well first I should touch on the ideologies of black metal, since they've been brought up. Black metal is not racist, it simply talks about ancient times where racism was prevelent; case in point, a biography of Hitler isn't racist material. Black metal promotes individuality and freedom and, if you wanted to use a political ideology, it would come closest to anarchism. Black metal therefore opposes religion and facism (big misconception that black metal is facist) since these restrict individual freedoms.
But black metal concentrates heavily on emotion. It seeks to carry the listener to another land, a cold, dark, technology-free, mysterious land. And that, IMO, is really the key to good music--emotion. Music can try to promote an ideology, but it can only do so much within the constraints of a song (as opposed to a persuasive essay, etc). All good music, however, attempts to evoke a certain emotion in the listener. The music's inherint ideologies can help the listener interpret the emotion one way or another, but the choice is ultimately individual to each person.
For example, I (and I know that many think this way as well) take black metal to be something positive. That is to say, it takes all these negative themes--death, satan, war, etc--and builds a magical realm on top of all of it. From the bare, primordial essence of life, we can create so much. Now notice that some bands, like Nokturnal Mortum, might promote racism or other misguided beliefs. But their words can only do so much, and the sound of their music, and thus the emotion it creates in me, takes priority and I'm able to enjoy their music according to my way of looking at black metal, not theirs.
Like Speed said, ideologies are individual but music, emotion, is universal. That is to say, all good music evokes a certain emotion in the listener, and this has less to do with perception. We then build on this emotion, creating our own individual views/philosophies that are influenced partly by the band's ideologies but mainly by our own individual views. Therefore, no, ideology is not very important for bands or genres of music to have. It is, however, important for a band to focus on a certain set of emotions, so that their music can come together to a point of focus as opposed to going all over the place. But, this is not ideology.
As a last little note, that's probably one of the reasons I think so highly of black metal--it focuses on emotion so much.