The question of "should I (Prof. Jr. Prof) review for journals? And how much?" is something that comes up all the time. It has, it does, and I suspect always will, come up in my meetings with jr faculty (as chair of promo/tenure/advising committees at various uni's). It comes up in casual discussions with jr and even sr faculty. It got asked of me by one of the tweeps in my online community.
I've edited & organized some of the answers I gave said tweep:
There are advantage:
First and foremost you are going to learn about what else is going on, in stuff close to you, on the ground floor. Yes, yes, of course you could read it after its published, but would you? At the same level that you review?
Secondly, you make friends (sorta kinda) with an AE. AE's always have trouble finding reviewers and when you do this, they like you. They REALLY, really like you.
Thirdly, you may learn something about writing by seeing stuff before publication. What needs to be changed. What you see as mistakes in others that you might have been blind to yourself.
BUT... There are many downsides, and almost all of them have to do with time.
Time, my friend, is your most precious commodity. It is more valuable than summer salary (if possible, but that's another post). You have much to do. We all have too much to do.
The advantages and benefits of reviewing are not linear with the number of reviews. These level out.
Do some, but a few a year. Protect your time. You don't get tenure or promoted for reviewing papers but for writing them.