Hmm... I have larger marbles and river rocks mixed as the substrate in one of my tanks. The others have plain white aquarium gravel. I can definitely not see the poo at the bottom as easily in the one with larger rocks... But it also makes it extremely hard for me to siphon out the bottom because the poo sinks into the rocks. Then again, I don't do 100% changes because it is cycled.
With the regular gravel, the poo usually sits at the top of the gravel, or sometimes on the decorations, so I have to look at in anyways, lol.