I use the following approach which works well in that the Valid method fires only when the user makes a change to the text box.

I set up a property THISFORM.changed.

In GotFocus:
THISFORM.changed = .F.

In InteractiveChange
THISFORM.changed = .T.

In Valid:
If THISFORM.changed
Check validity of data
ELSE
Return
ENDIF

In LostFocus
THISFORM.changed = .F.