This is not a matter of taste. This is quantifiable: Upscaling bitmaps incurs resolution loss and thus is a design no-no. Yet this is how the parallax and folder effects are achieved. Rather than being meticulously hand-rendered in Photoshop (the Forestall method), iOS 7 is mostly vector-based, allowing for unlimited scalability. Which is good. Great, in fact (think 20 different iPhone sizes). But until we have vector background images as well, the workaround is to require images larger than the screen area with a specified minimum amount of wiggle room. The irony is when you provide a large enough image it still stretches it (this isn't a bug, it is an intentionally defined value with a known location).