I have my tank right up against the wall. Zero clearance. It's recessed into a wall actually and I've left nothing but .5" for clearance on one side - just enough to sneak the cords in behing the side of the tank. I wouldn't do it any other way for this house but having a set up like this limits me on a few things. I had to do without the vortech's for example unless I wanted them on my viewing pane of glass.
In my next house I will have a set up like some of the mnoster tank guys here on canreef - with its own sump and quarantine room in behind the tank so I can have infinate clearance for whatever you can imagine.