Validation over-ride on Subforms if the Subform is not needed?

[cw via AccessMonster.com]

I have placed validation on my Main form fields as well as my Subforms fields
using this kind of code:
-----------------------------------------------------------------------
Private Sub DialUpRatePlan_Exit(Cancel As Integer)
If IsNull(Me.DialUpRatePlan) Or Me.DialUpRatePlan = "" Then
MsgBox "Please choose a Dial-Up Rate plan from the drop-down list",
vbExclamation, "Required Field"
Cancel = True
End If
End Sub
----------------------------------------------------------------------

It works fine & requires a User to enter data before proceeding on to the
next field.
The problem arises on the Subforms. All of them have my validation code on
required fields but not every Subform is required to complete data entry.

At the end when I close my Form or try to Save it, I get prompted for all of
the Subforms that were passed over. (Data will always be entered on the Main
form, but only on a few Subforms at any one time)

Thanks for your help,
cw

How do I tell the