I always do my undersides. First I spray grimy black, then rust. I do them pretty much the same as I do the trucks. An example of a caboose and a flat car.
I haven't really weathered anything yet, but if you don't weather the bottom it's going to look weird when you have a wreck and they are all shiny on the bottom.
While I have weathered a few on the bottom, it's not a standard procedure for me. I have mainly done it on old Roundhouse cars with metal floors. I do weather many air brake reservoirs that can be seen from the side.