The servo grommets are always worth fitting whether nitro or electric, even though electric doesn't have the operational vibration from the I/C engine, vibrations cans still be present or manifest themselves at a later stage so it is always worth installing the grommets to protect the servos and keep them isolated.
I have always fitted the servo grommets on all my machines, nitro or electric. They are cheap insurance in protecting the servos. I would replace the servo case and install the grommets on all the servos.
Hope this helps,
Cheers,
Ross