What's the best and safest practice for releasing outlets?

-viewDidUnload is strictly used for releasing IBOutlets with retain properties.

The reason for this has to do with the fact that UIViewController has a view property which it retains. That view property itself retains references to all of its subviews. These subviews are exactly what you are retaining inside these outlet properties. The problem lies in that these subviews have an "extra" retain on them.

The goal of -viewDidUnload is to clear up unnecessary memory usage. When -viewDidUnload is called, the view property has already been released, which releases the top level UIView along with all its subviews. Since we have retained some of these subviews however, they linger in memory, and we want to release them since they will no longer be used. New copies of these subviews will be created when (if) the view is reloaded. The properties are also set to nil, strictly so we don't have pointers pointing to deallocated memory.

In -dealloc all retained properties and instance variables should be released. In the case where the -viewDidUnload just executed, you will be sending a harmless [nil release]; to the IBOutlet retained properties you just set to nil.