I have mine stacked so that some of it touches the back glass but not everywhere. A good thing to keep in mind is to have a good size area for a cryptic zone. This will provide a good environment for anthropods and copepods to breed.
I also don't clean the back or the sides, this is because I want it to be coated in Coraline Algae.
It is also a good trick to use plastic electrical quick ties to brace rocks together. Just work with exsisting holes to tie them together or you can drill a hole.