Currently emails go out to all participants in a review. However, there are two very common scenarios in which emails are sent that are annoying and don't provide any extra value.
1) When I create a review I get an email that says "You are a Author for new Review #XXX". This should not be sent, I know I am the author of the review.
2) When I am the reviewer that completes a review I get an email saying the review is complete. This isn't necessary, I just completed it. Now I guess you could have a race condition where two people are reviewing and click Approve at nearly the same time and so you might not realize you're approval was actually ending the review, but I would argue for this to happen no one made any defects or comments so it doesn't matter you still don't care. But at a minimum if there is only one reviewer, and they click approve, they don't need an email.