Should I agree to review papers as a postdoc? I am a post doc in mathematics, and was just sent a request to review a paper. This is the first time it happened to me. I never heard of this journal before, but it is a Springer journal, and upon checking the editorial board it looks like a respectable journal. Also, the paper looks quite interesting, so it appears that I shouldn't worry about it being dubious.

I wonder however, if this is a good time in my career to do such a job. Given that I am a postdoc, constantly in the run for the next job, and probably this will take some of my valuable research time. Is it normal for postdocs to review research papers? Should I accept it? If so, should I mention the fact I review for this journal in my C.V?
Yes, reviewing papers is an integral part of your job now, and a very beneficial one at that!

It's true that the first papers you will review will take you quite a bit of time, but it's a sound investment (unless you consider dropping out of academia next month). You'll improve your reading and ability to read between the lines. It will help you gain a wider view of what others are doing in your field, because it will force you to really go through the paper in depth, not just skim over it as we are so often forced to do when a paper is not at the core of our own research.

Moreover, it will give you invaluable experience when you write your next papers, because you will be able to put yourself in the reviewer's shoes! You will see better what could be problematic for a referee in the presentation of your work.

As for whether you should list it on your CV: yes!