I personally put eggcrate down on the bottom of the tank, than start with my larger "base" rocks.....I will literally only set rocks wherever they "fit".....You know when they fit because they don't wiggle or wobble. Use at least 3 points of solid contact for every rock. I don't attempt fancy caves and ledges unless the rock gods give me the optimal setup with 0% chance of failure.....
My current "collection" of rocks is not very impressive either, which is why I literally have a pile of rocks....In a few years when I upgrade for the last time, I will be out looking for very specific shapes and sizes to accent the stuff I have.
Always keep a good base in solid contact with the bottom of your tank or eggcrate. As others have commented, certain livestock will move sand. Sand can shift over time, and even does dissolve.