To add to this, even though you've already got a perfectly good solution, I use Shutter to do this.
I first remove the default print screen software, regardless of distro.
I then set the shortcut keys of Print Screen to take a full screenshot (command shutter -f) and Shift + Print Screen is set to selection, which is shutter -s.
Why? Because Shutter has a nifty upload function and can be cropped and edited from within the same application. It lets me draw arrows and text, all from within the Shutter application.
This is admittedly bloated and more than necessary - but Shutter is remarkably light and my personal use means that it's actually open anyhow, or will be open eventually.
This being Linux, there's more than one way to skin a cat.