Short answer - if your UICamera is set to have an eventType of UI, you don't need to worry about it.
Long answer:
NGUI 3 uses RaycastAll to collect input hits when working with an event type of UI, meaning it's able to see _all_ the colliders underneath a touch/click rather than just the closest one. It then sorts these by the depth of its panel * 1000 + its own depth. In practice this means that depths are all sorted by the same depth as they use to draw.