Hardware store variety flat washers are stamped (probably in a factory in China) and are not typically real flat nor real uniform in thickness.
I'd personally never use a stamped washer on any rotating anything.
Machined face washers can be purchased by the box at places like McMaster Carr. Bearing spacers or bearing locking collars are also readily available from multiple sources.
The amount of run-out at the end of a 20" blade could be pretty substantial using cheap washers, particularly if several are stacked up. Really not a good practice.