It doesn't appear that there is a registry entry for this.
You could create a class module in the user's Personal.xls file (or in any hidden workbook stored in the user's XLStart folder)
You can find most of what you need here:
http://www.cpearson.com/excel/AppEvent.htm
Open the Visual Basic Editor in Excel by going to Tools -> Macro -> Visual Basic Editor. Locate
VBAProject(Personal.xls) on the left side of the VB editor, click on it and then go to the Insert menu and choose Class Module and then paste the code below:
Option Explicit
Private Sub App_WorkbookBeforePrint(ByVal Wb As Workbook, Cancel As Boolean)
Application.EnableEvents = False
ActiveWorkbook.PrintOut Copies:=1, Collate:=True
Application.EnableEvents = True
Cancel = True
End Sub
Then double-click on the ThisWorkBook module of VBAProject(Personal.xls) and paste this code:
Option Explicit
Dim AppClass As New EventClass
Private Sub Workbook_Open()
Set AppClass.App = Application
End Sub
Click on the Save button in the VB editor. Then close and relaunch Excel.
Note: This will force the printing of all worksheets all the time, even if the user chooses to print only a selected area or activeworksheet