The most recent info I got at my camps this summer was that the Center now has a "bigger dance floor" and can work the entire width of the free throw circle. In general, the teaching this summer was that you work a high C (a couple steps higher than the free throw line) if the ball and at least two matchups are above the FT line. The reasoning being that the action that might come your way (ball matchup or pick) is more likely to be high. As the ball goes below the FT line, you would probably want to step towards a low C (a couple steps below the FT line) for the same reason.
That being said, there is no golden rule that works for all situations so it's a general guideline and not a hard and fast rule.
As far as stepping up or down, I believe that you just do what puts you in the best position to get an open look on your matchup. Sometimes it's up, sometimes it's down.
