Should I maintain relationships diagrams in the back-end and the front-end?
I have never used the relationships tool for anything other than understanding the relationships between base tables on the back end database.  I could see however where it would be very useful on the front end if you have many users of your application who write a lot of ad-hoc queries and you provide them queries stored on the front end database which they can use as an abstraction layer.  On the other hand, if the users only interact with the database via forms I would think the work to add the queries to the relationship tool and connect them up would not be worth it.