Hi,

The motor isn't in any danger but anyone infront of the motor when it is running may be. The washer is to spread the load from the nut to ensure pressure across the widest area to both hold the prop on and stop potential cracking of the propeller hub. Without the washer the forces are concentrated in a very narrow area of the prop hub immediately behind the prop nut.
Many years ago I had a wood prop on a control line combat model that was too thick at the hub to allow the washer to be fitted, so I just used the nut. One day on starting the prop fractured at the hub and stuck into the ground immediately below the model.

Your Choice,

Colin