No need to create separate xib files for iOS6. You need to set the UI for iOS7 as well as iOS6.
- Just open your storyboard and then click on assistant editor.
- Now open storyboard preview in assistant editor.
- There is a button in assistant editor at right hand bottom corner which says iOS7 or later, click on it and select iOS6 or earlier.
- Now you can see the UI for both iOS7 and iOS6 side by side on the screen.
- Now select a particular view controller and then select size inspector.
- In size inspector you just need to set the iOS6/7 Deltas for iOS6 screens. That's it.
Main UI difference in iOS 6 and iOS 7 is that status bar is included inside the viewcontroller in iOS 7. it means your view controller is 20 px greater than iOS6. you have to adjust your items. Design your items according to iOS 7 and set Δy to -20
check this url too : https://developer.apple.com/library/IOs/documentation/UserExperience/Conceptual/TransitionGuide/SupportingEarlieriOS.html
If you have any problem then let me know.