The grommets give the servo some isolation from vibration on models using internal combustion (IC) engines. Most people don't use them on electric powered models.

The servo should fit slightly loose if the grommets are used, otherwise they would serve no purpose. Tighten the screws down just enough to hold the eyelets, but not crush them.

You may find that mounting the servo securely without grommets gives better control, and choose to omit them even in IC-powered models, despite the negative effect on servo life.