Client async callback

 I am using async client callbacks and have two questions :
1) do I have to call GetCallbackEventReference, etc, every time the page instantiates, or can I safely bracket it with an if (!postback) {}
2) Why are the local variables not persisting? 
 
 
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
if (Request.QueryString["AccidentID"] != null)
{
this.insertMode = false;

}
else
{
this.insertMode = true;
}
Session["insertMode" ...