The manufacurer recommends isolation mounts and goes to the trouble to supply them with every servo.  I have never not used the grommets and stand-off bushings, so I can't say the teeth will all fall out of the servo simultaneously and at the worst possible time if you don't. 

I take it this is an electric model?  Perhaps direct mount is good enough.  Some just stick the servo in place with two sided tape.  I wouldn't do it on a model with an I/C engine.