For Edexcel, you will have to write about your non-core ideology in your second paper. Unless you want to learn something else like feminism or anarchism, you will have to write about nationalism in that so you will need to know it - you do not get a choice. As for the core ideologies, yes, you do get a choice of 2 questions but there's nothing saying it can't be like 2 questions on socialism, meaning if you didn't know it you would be at a massive disadvantage.
If you have been told it's not coming up in your mocks, don't revise it. But if you haven't been told that, at least understand the basic concepts behind it, and for the real exams, make sure to know it