They are best used to maintain some amount of cushion effect in the rubber. If you leave them out, you can tighten the screws down to a point where the rubber has no vibration dampening effect. My experience with Futaba servos has been to install them flange down, the shoulder of the screw comes down on the tube end of the bushing and forms a grommet to hold the rubber tight, yet allow it to dampen vibration.

They also help maintain grip on the servo. This will keep your servo from becoming detached in flight! A most challenging predicement!