There are no "rules" when running Linux. You run Linux how you need it ran, not for status. If your looking to build a server that you will SSH into and run some services, than the CLI will suffice. If your looking to add multiple programs, and use every day applications but still need the CLI on the side, the GUI is perfect.
We really need to look at the functionality of the OS, not the 'status' or 'rules'. No such thing as breaking a law or committing a sin for building out a Linux box to fit YOUR needs. After all, you're building the box for yourself or a client.
If a Linux guru gives you grief for installing a GUI and not using full CLI means nothing. Think about it, if you ask him to run a firewall with full CLI or configure a BGP router from scratch ...or even code a specified driver he'll be stumped. Why? Because everybody has strengths and weaknesses. He might only use CLI in Linux, but has to use GUI on firewalls ect ect. You will never find a gEEk who knows it all, if you do send him my way. I would love to meet him.