I guess it all depends on what you run and the trackage conditions?
In my case operating 3R True O, there is no need to add extra weight to rolling stock. My freight car MAX limit is 1.53 lbs. which would only be on the Atlas O Master 50 footers. Atlas O trucks are heavy for 3R. The ideal / average freight car weight in my operation is .95 oz. which is based on Lionel PS-1 box car series with box loads.
MTH had to make the power unit heavier on the MU sets to pull add-on cars. All the MTH diesels are perfect. On MTH 20- Gondolas, I removed the black metal plate underneath, no need for it.
On Weaver, all my cars contain die-cast T&C except cabooses. I cut the Weaver center weights in half and RTV them over each truck. Running long trains, weight is a factor.
As my Dad taught me, “heavy cars up front, lighter ones toward the back”. That’s a sure way of operating long trains trouble free!