Should I agree to review papers as a postdoc?
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!