You need the To in the file that it is used. So you'll need them in the interface as well as the data files.
However, you do not necessarily need all or the same relationships in each file.
The relationships in the data file should be those needed for calculations whereas those in the interface file those needed for display purposes.
So you may not need a People -> Tel relationship in the data file as it's never used in any calculations, although it will be needed in the interface file.
Hope that helps
Yes. Every file must have its own Relationships Graph, including all Table Occurrences needed for whatever functions to be performed.