u will stack and rearrange MANY times before you find a combo u like. u may have to also rearrange for other reasons (wanting the fish to re-territorialize, etc). thats why i would suggest not using epoxy to glue rock together. find a way they fit stably.
anyway, u DO want some room in back for the fish to have cover and a place they can hide behind. thats why tunnels and holes are so cool. also, leave room to catch fish in case u need to remove any for treatment, etc.
some can go against the back if ur trying to hide something, like a built in overflow.
most of all though, HAVE FUN WITH IT! the tank is now a living piece of art in ur home. tank ur time (pun intended) and see what appeals to your eye.
oh, and remember, pictures are two dimentional, so what looks like its against a wall may not necessarily be.
HTH!