Best practices for UI asset creation while using NGUI.

Hi,

I am working on a game for IOS and Android phones. I have to use NGUI to create the Hud elements fo rthe game. I have created the graphics using illustrator. Can someone guide me on the best practices while creating assets for Ios devices using unity? Is their graphic loss during scalability? Can AI files be used or should I save them as PNG files?

This is the first time I am using UNity and have to learn the software while implementing the game, so any help would be great!