How long does internal bleaching last on root canal? Will I need veneers to cover it up?

I recently had a root canal on my front tooth and had internal bleaching done since it was slightly grey towards the top. Now, I am in the process of leveling out my front teeth. I would rather have composite bonding done on my other front tooth so they can be the same length. However, my dentist recommends a veneer on both front teeth since they believe the root canal tooth will eventually revert back to its old color. Is this true? Which is the better option?