Courtney Starr of Savvy Apps writes,

The iOS 7 OCD app icon template is different than others in the fact that each app icon is its own independent smart object. This allows you to design your app icons at the exact sizes that they’ll be used. Another huge difference in our template is that the grid is uniquely crafted for each app icon. Thus, yielding pixel perfect values at every size.

You can grab the template from GitHub.