Wow ... it seems automatic.
You actually have to scroll the Bin Slider to line it up just right before dropping it. In that first picture you can see the lettuce bin is slightly off-center to the left, so if you dropped the lettuce like that it wouldn't be lined up perfectly for a good score. You can make some really sloppy burgers if you're not careful on the Build Station

Here's a shot of a sloppy burger, where I wasn't careful lining things up. This one's in widescreen too, which is how it looks on iPhone 5 or Android phones:
