I typically add rollers to upgrades that have only two closely spaced rollers on the locomotive. Here's a couple of examples. As others have said, sometimes with MTH stuff, the mounting hole is already in the truck so it's just buying the pieces to mount them.
Here's a couple that I've added after the fact, I run into this a lot working on brass locomotives.
Here's a different twist, this was an MTH switcher that had two rollers on the tender and none on the locomotive! It had big problems with switches!
