Ideally you want the tweeters of your LCR speakers to line up, or as close as possible. Tilting the centre channel upwards slightly can help as usually people have it set up under the screen/TV. Axiom speakers are tapered cabinets so placing your plasma on top of the centre channel would be a no-go. Even if they were parallel I wouldn't do it anyway(if it were me).

Half of communication is listening. You can't listen with your mouth.