Sometimes we might joke about this issue, but it is an important step that many modelers neglect. A few years back I ignorantly assumed that the "cheaper" locomotives, such as Athearn Blue Box units, required complete "porch to porch" mechanical servicing. That is, until one of my trusty Atlas FP7s (newer version) started to run a bit on the noisy side. Turns out it needed lubing and the factory grease had dried out. Mechanism needed cleaning too.
I'm getting ready to install DCC and DCC/Sound in a good number of my units so they will be partially or fully disassembled for a thorough inspection, cleaning and lubing with Labelle products.