With all the vibrations occurring in cars it's a wonder that the electronics work at all. Certain things just don't go well with electronics.... water, heat, and vibration.
I'm all for using the rubber grommets, but only in conjunction with the brass eyelets that are placed inside of them. If you can eliminate the vibration to the electronics in the servos, you may find they last longer and protect the gears better. When using the eyelets in the center of the grommet, the servos don't have much room to move around if everything is secure.
Using only the rubber grommets, your servos will definitely have some slop.