This is just me speaking from memory but I think in the wild Crochea clams are commonly found in rocks. They have the ability to dissolve rock so they can wedge into a crevise for protection. I don't think maximas can do that so in the wild they tend to be in the sand. I have both of my maximas on rocks and they have attached themselves with their bissal strands and they are pretty stable.
I also have a crocea clam that I put on my shallow sandbed. After, I read that they typically live on rocks I tried to move it but it had attached its bissal strands to the glass bottom. I just left it and it is doing fine.
It is my opinion that in the aquarium it does not matter where they are located as long as they have adequate light, can attach themselves securely to something and they don't get covered by sand easily if they are on the bottom.
