ActionScript 3.0 :: When To Use Primary Expression Keyword (This)

I'm a complete novice to ActionScript, OOP and Flash. I have no idea when you are supposed to use the keyword "this". I have searched the forums but obviously searching for the word "this" is not very successful! I've read quite a few tutorials now and it seems that different people have different opinions on when and where it should be used. For example what should I be doing in the following class? Both seem to work but is one the "correct" way? Are there situations where things will not work if you omit the "this"?

ActionScript Code:
public class Sector {
private var _radius:Number;
public function Sector(radius:Number){
[B]this.[/B]_radius = radius;
} public function get radius():Number{
return this._radius;
} public function set radius(value:Number):void{
this._radius = value;
}}

I have another possibly related question with regards to naming conventions. I like the use of underscores for properties with getters and setters but what about the names of the constructor parameters that are used to initialise these properties? In the above example I have used "radius" for the constructor parameter although it is also used for the getter and setter functions. It seems a bit of a palaver to come up with different names for the same quantity but is what I have done really bad coding practice?