Sup Scott, as you've seen, I always place mine on the sandbed since I like to save the rockwork real estate for the sps.
As far as mantle extension goes, rockwork or sandbed, won't make or break anything. In the wild most clams are found on rock, but like I said, it's not a big deal either way. Go for whatever looks more pleasing to you.
Email me some pics of your setup too man. You know my email.