Kostas,
We have not had any feedback from students who have taken the PMP exam in this regard. So I would have to say that it's an optional read.
However... be aware that that it is an official PMI global standard, and that questions for the PMP may come from many sources. But the big consideration is of course: How much time do you need to invest studying it <---> the actual chance of getting a question based on the standard on your exam.
My suggestion: If you already own a copy of the standard, then do take 2-3 hours of your time to skim through it and get a high-level understanding. If you don't own it, then don't worry about it.