Pick up the Pink Floyd DSOTM SACD. If you not hearing your center channel, or your sub for that matter, then something is up with your playback system. Also, it is very much a reference SACD worth picking up.

As for other SACDs, it will be different from disc to disc on wether there is a center channel or not. It is completely the engineer's/producer's choice of wether or not to include it.

For more info on your discs, try these sites:
www.sacdinfo.com
www.highfidelityreview.com