Chad I do a lot of Freight and Passenger car scratch building I try to put as much detail as I can into each car. How ever when it comes to Flat cars and Gondolas that have fishbelly sides that completely hide any under body detail I leave it out, except for the center sills and cross members. So, my bottom line is if you can't see it I don't model it. This leaves me more time to do the fine detail that you can see.
