Corner shape of modal view on iPad in ios: square or rounded?

Is there a recommended corner style for modal views (not popups) on iOS? Is the square style preferred or the rounded style? Does this recommendation differ in different iOS versions?

I don't see an official Apple recommendation in their user interface guides. The App Store modal popup to view the app on iPad has square corners on iOS8. Twitter for iPad iOS8, for example, has a modal popup with rounded corners.