I would have thought that every time you place a say cube on the level for it to have its own material, as it is when you change the textures you also have to manually change each cubes material.
The video below shows a collection of cubes making a corridor and a room, changing any ones texture changes it for every wall, floor, roof...
A more common approach would be Cube_1, Cube_2, Cube_3 etc and each new cube in the scene gets its own material, such as Cube01_material, Cube02_material, Cube03_material, etc..