Commodore 64 software hardly ever uses the NMI interrupt, because there is also the maskable IRQ interrupt which can be generated by the first CIA chip, and also, unlike NMI, by the video chip VIC for the raster interrupt. It is also much more useful because it is maskable - the software can temporarily turn it off when it is not desirable to be interrupted. The only thing the NMI can do that IRQ cannot is detecting that the Restore key was pressed, but that is seldom useful.
