the last washer is needed to keep the steps at a full step. by going to the 5th groove and leaving out the washer you are at step 4.5 on the needle.
All manufacturers have their instruction for jetting on the rich side. It's a liability issue, jet too lean and you can damage the engine, jet too rich, all ya get is lousy gas mileage, fouled plugs and excessive carbon build up.
The effect of the pilot
ckt fuel on the main
jet circuit is negligible as far as the over lap of circuits. As throttle increases, vacuum decreases and less fuel is drawn thru the pilot ckt. As well as the main jet plays a very small role in cruise mpg. Cruise speed as per Mikuni is 65mph. I tested this by going up in main jet to a 190 when I normally would have a 167.5, but by not getting heavy handed on the throttle, my cruise mpg did not drop anymore than 1 or 2mpg. Theory is theory only in a fully controlled testing, but does not hold 100% true in real life.