I think it all comes down to personal taste really. I think knowing mainly what coral you are going to house is a big thing. Liek I knew I wanted a toadstool, so I left the left side of my tank fairly empty for him. Also again as I think someone said up top is setting up your rock so you create depth inside your tank, with big tanks depth is so easy to create but with nanos I think thats where most people struggle with the rock work.
Heres my tank but you cant see much rock
Heres my tank but you cant see much rock