iOS 5 Best Practice (Release/retain?)

As a beginning iPhone programmer, what is the best practice for writing apps to be used either with iOS 5 or older versions? Specifically, should I continue using the release/retain of data, or should I ignore that? Does it matter?

performSelector may cause a leak because its selector is unknown

I'm getting the following warning by the ARC compiler:

"performSelector may cause a leak because its selector is unknown".

How do I replace weak references when using ARC and targeting iOS 4.0?

I've begun developing my first iOS app with Xcode 4.2, and was targeting iOS 5.0 with a "utility application" template (the one that comes with a FlipsideViewController).

Minimal requirements for ARC using

In apple documentation I saw that minimal requirements for ARC is iOS 4.3, but on WWDC 2011 video they talk that minimal requirements in iOS 4.0.

ViewController gets deallocated which leads to crash

I have a view in my storyboard which I assigned an identifier called "MainView". However if I add its view to the subview, everything that follows produces a crash (e.g. pressing a button)