At what point is it OK to claim that you review for a journal in a job application?
I am not sure how this would be considered in a PhD application, but I think it would strengthen my profile, given that this will be the second most prestigious journal to which I will have provided a review.


While it doesn't influence the when, I am not sure your premise is correct. There is something important about an editor asking you to review for a journal since it demonstrates that someone (specifically a respected someone) thinks you have a thorough understanding of a field and are capable of independent critical thought. The act of reviewing is not really worth much.

As for when you can disclose it, I think this is a little messy. I would probably wait until the editor sends a decision letter to the author. At that point, you have clearly reviewed for a journal. Prior to that, I might mention that I have agreed to review for Editor X at Journal Y.