Sticking to the perimeter of the typical chain grocery store is definitely the healthiest route. Although that doesn't mean its void of processed/packaged foods but they're definitely out numbered by real food. In addition to the center aisles being a minefield of highly processed foods, the checkout is another treacherous spot. Even Whole Foods and Trader Joe's are guilty of the strategically placed, packaged, impulse buy items such as organic lollipops and boxed cookies. That's not saying I am against organic lollipops because I consider them a treat and a "Medium Food" but I am aware that its placement is a standard supermarket tactic.
Here's a great little spot with Michael Pollan about navigating the typical supermarket. He briefly explains where and why items are placed as they are at the grocery store. It's good information and interesting too.