I have always used 3M Super 88 electrical tape. It's inexpensive, just thick enough, durable and very easy to replace. Having replaced several grommet and bumper guards, I wouldn't want to have to do that very often.
I disagree with others who say the bumper guard serves the purpose as my son has cracked 2 bumper guards on Head Instincts. Maybe these were just brittle plastic, but once they crack they are of little use.