Should one put poster presentations on their CV in maths? I have presented posters at poster sessions at a few large conferences in pure mathematics. Is it worth putting these down on one's CV? 

For context, I am a PhD student and have also given talks at several seminars and (smaller) conferences, so my CV is not especially in need of bulking up. Currently I do not have them on my CV. Moreover, my feeling is that posters in pure maths are not peer reviewed at all (in contrast to my impression of other areas), though I do not know this for definite.
Given that you are a PhD student (in math; I am also in math), I think you probably should put this on your CV.  Going to conferences shows some professionalism and interest, and giving poster presentations does too.  I don't want to suggest that this will be a key piece of information for your job search and/or early career, but...it certainly can't hurt.  You may want to take them out later when your CV has filled out in other ways.

The fact that they are not peer reviewed does not seem directly relevant to me, as long as you don't lump them together with publications (why would you?) or suggest in some way that they are something they were not.