The sample shows how to use the SavingReport and LoadingReport events in the code. You can add the event listener to the StiDesigner class at application initialization:
static Form1()
{
	StiDesigner.SavingReport += new StiSavingObjectEventHandler(OnSaving);
	StiDesigner.LoadingReport += new StiLoadingObjectEventHandler(OnLoading);
	
	// If you need you can hide menu item "Save As.."
	StiMainMenuService service = StiMainMenuService.GetService();
	service.ShowFileReportSaveAs = false;
}

Specify the action to be performed when the report will be saved:
private static void OnSaving(object sender, StiSavingObjectEventArgs e)
{
	StiDesigner designer = sender as StiDesigner;
	//string str = designer.Report.SaveToString();
	MessageBox.Show("Report saved");
}

Specify the action to be performed when the report will be loaded:
private static void OnLoading(object sender, StiLoadingObjectEventArgs e)
{
	StiDesigner designer = sender as StiDesigner;
	//designer.Report.LoadFromString(str);
	MessageBox.Show("Report loaded");
}

In the screenshot below you can see the result of the sample code:

Saving and Loading a Report in the Designer