How do I uncheck the recurring contribution check box by default?
You're in the right place to see what's going on, but the wrong spot to do anything.  The field will arrive in the template fully-rendered, within {$form.is_recur.html} or an equivalent Smarty variable. Editing the template won't help you there.  

The form PHP code would allow you to change it, but that's not the best way to go about this: doctoring core CiviCRM code is a bad idea, and while there's a way to set up override files for code files, it's a clunky method.  Besides, you'd have to navigate Quickform, which is a headache in itself.

The better approach to forcing a checkbox checked or not is to use jQuery.  You'll have CSS-style selectors, so you won't need to troll through the templates to see where to alter the box.  You can add your code in one of three places:


Somewhere in your WordPress theme
In an extension you write, using the Resource addScriptFile method to add a script within hook_civicrm_buildform (the preferred method)
Using an .extra.tpl file in your template override directory