Flex 3 - Must I add components before setting their attributes when using AS3?

var a:A = new A();
a.b = "woopy"; //Sets the Label (declared in mxml) withn A to "woopy"
this.addChild(a);

<customNamespace:A b="woopy"/>

var a:A = new A();
this.addChild(a);
a.b = "woopy"; //Sets the Label (declared in mxml) withn A to "woopy"

var a:A = new A();
a.initialize();
a.b = "woopy";
this.addChild(a);

<?xml version="1.0" encoding="utf-8"?>
<mx:Canvas xmlns:mx="http://www.adobe.com/2006/mxml">
    <mx:Script>
    	<![CDATA[
    		[Bindable]
    		public var labelText:String = "[Default]";
    	]]>
    </mx:Script>
    <mx:Label text="{labelText}"/>
</mx:Canvas>

var a:A = new A();
a.addEventListener(FlexEvent.INITIALIZE, a_initialize);
addChild(a);

private function a_initialize(event:FlexEvent):void
{
    a.b = "woopy";
    // ... and so on
}

private var label:Label;

public function get b():String
{
    return this.label.text;
}

public function set b(value:String):void
{
    this.label.text = value;
}

private var label:Label;

private var _b:String;

public function get b():String
{
    return this._b;
}

public function set b(value:String):void
{
    this._b = value;
    this.invalidateProperties();
}

override protected function commitProperties():void
{
    super.commitProperties();
    this.label.text = this._b;
}

<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml">
    <mx:Label text="{this.b}"/>
    <mx:Script><![CDATA[

    private var _b:String;

    [Bindable]
    public function get b():String
    {
        return this._b;
    }

    public function set b(value:String):void
    {
        this._b = value;
    }

    ]]></mx:Script>
</mx:Application>