Since I spent some time today working with control extenders and exactly where in the page lifecycle to do what, I thought I'd share the whole list.
If (!IsPostBack)
Begin PreInit End PreInit Begin Init End Init Begin InitComplete End InitComplete Begin PreLoad End PreLoad Begin Load End Load Begin LoadComplete End LoadComplete Begin PreRender End PreRender Begin PreRenderComplete End PreRenderComplete Begin SaveState End SaveState Begin SaveStateComplete End SaveStateComplete Begin Render End Render
If(IsPostBack)
...
Go to the complete details ...