Juniper Monitoring

What ports should I monitor on a Juniper router?  I have a J4350 that is connected to our LAN via Ge0/0/0.0 and Ge0/0/3.0.  The router is connected to the MPLS via T3-1/0/0.0.  Should I just monitor the physical connection, meaning the 0.0 and 3.0.  I am currently monitoring both the interface and sub-interface and notice that the bandwidth utilization shows different values.  I also wanted to know, if any, which volumes on the routers I should monitor.

Thanks,

Damien