No doubt, over time rods can slip a bit but , if that is the primary cause of an out of tune drum, then why do drums go out of tune overnight? You can tune a wood shell drum so that each tuning position is dead on the same note , leave the drum for a week unplayed and it will be out of tune, in much less time if the weather changes dramatically. I agree that nylon is a poor choice for washers , in this application but then so would any sort of lock washer. Drums are made to be tuned , so they can be tuned----just like any other tunable musical instrument.
Put a fine marker line on the t-rods down to the hoop of a snare drum and play it for a week and see where those marks are relative to the pitch at the tuning positions.