To park your clam(s) on the sand or live rock is largely a question of two things 1) clam type and 2) light parameters. Some clams don't need as much light and some need more. Some prefer rock to attach to and some prefer sand. Light being the most important of the two.
And sometimes, you don't have much choice as clams can and sometimes will move to a new location if they are not happy with where they are.
Maxima's and croceas tend to need more light and prefer to attach to rock, so generally better to move them up in the tank on rock, or at least maybe start there.
Deresa, squamos, and gigas tend to need less light and often prefer sand. They can usually be placed on the bottom on the sand bed, assuming adequate lighting.
These are VERY general guidelines to maybe start from. Again, it depends largely on your lighting system.
Supplemental feeding (phytoplankton, zooplankton) will also help tremendously, and may be critical if light is on the inadequate end of the spectrum.