I have trimmed just a hair off the trailing edge but no more than what is normally discolored from arcing. I can't say if it really helps since I have just started trying it.
Something else you can do, is after a run set the car up and run the motor on 1.5 -2 volts and hit the comm with a couple shots of motor spray. This will clean the brushes and comm real well without having to remove them and keeps them seated to the comm.
Greg