Tropheus will spawn on the tank bottom, gravel, a flat rock or a slanted flat rock. My Kalambo colony had a dominant male that preferred a slightly tilted flat rock, a slate roofing tile. This act may be male specific, with different males preferring different substrates but I never really paid attention as to preferences.
They usually pick a place, dominant males that is, and females join them. It can be in a corner of a tank, under some rocks, or on a flattened surface. The dominant males will pick up their spots and interested females will join them.
They will even spawn on the glass walls (if thats all the male of the females choice can secure) or on the glass bottom in tanks with no substrate. I think the females prefer a male that can secure a flat level rock though, if there is one.