Using Fields as Attributes in Node Templates
Is there a reason to not just set the format to "&lt;hidden&gt;" in the content type's "manage display"? Then it will never render but you still have it if you want it. 

If you really want to do this programatically, the best practice, for me, is to use a module not your theme. Why? Because if your theme changes... do you want extra to show? Nope, so make it independent of the theme by putting it in a module.