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Well, not to diminish Paul's teachings but IMO, while it never hurts to be that cautious, it honestly affects nothing getting fingerprint oils or body salts onto the flywheel or Pressure Plates surfaces. Let's think in this light...
To begin with, oil rarely causes the Clutch to to chatter. It can cause it to slip however, -but then as soon as the surface oils have been overcome by friction (-i.e. heat), the issue is gone. This would be the case with body oils, etc.
So what causes 'clutch chatter'? As mentioned, it can be an alignment issue however generally speaking, the gyro effect of a rotating mass always seeks a centerline which masks the alignment issue. So in other words, the chatter is usually light & subsides quickly.
The biggest cause of chatter in a Model-A driveline from my experiences is due to a worn Clutch Pilot Bearing, or a worn end of the Main Drive Gear (-a/k/a Input Shaft) where it enters the Pilot Bearing, or a worn Main Shaft Bearing. Any one of these (-or combination of the 3) allows the Clutch Disc to be outside of the Pressure Plate centerline. When the Clutch is engaged, it is trying to seek a centerline, and therefore causes chatter until it becomes centered with the P/P. It was mentioned that the chattering does not happen once at operating speeds and during shifting, ...which this is because the Main Drive Gear (-Input Shaft) is already rotating within the centerline and the gyroing effect basically maintains that centerline of the input shaft and clutch disc even when the pressure of the Pressure Plate has been momentarily released.