I think you can pretty much just ignore the first couple of responses to your question...it's not selling out unless you're speeding it up just to make more money out of it. But if you're just writing it slower to make it easier on yourself, that's completely valid, especially if there's a complicated part that needs some practicing to get, but you want to go ahead and finish the song. And of course, using contrast, as in the Good Morning Vietnam example, is great for making your point more vivid. In that example, using optimistic music makes the violence even more shocking, and contradicts the mindless optimism of the song (though I do like that song) and the mindless optimism of the Americans going into the Vietnam War (sorry to sound like a college term paper). A similar example, but in song, is "Wave the White Flag," by Elvis Costello, in which the narrating character sings about abusing his wife to an upbeat country tune. The happy music helps show the stupidity of the narrator, and kind of parodies country tunes about the nagging wife at the same time. So I guess my point is, having something sound "out of place," as virtual.ray says, is not necessarily a bad thing, depending on what you're trying to do. Personally, I think those contrasts can very appealing.