if you make a pile in the middle it will eventually hit the front or back when doing aquascaping or general cleaning.. might as well decide which you want scratched up. Dense heavy rock might even break it when falling.. (thin glass like a 29 gallon or smaller is more likely) I had some heavy duty tonga branch punch a hole in a tank before and a few I droped a rock in that broke the bottom.. not pretty. Acrylic will be scratched almost instantly when rocks enter the tank, so be careful with those. In well over 100 tanks I have dealt with only a small handful did not have the vast majority of the rocks against the back wall. Cannot think of any other than a few cubes that had no rocks against the walls, but those had the rock up against the center overflow....