You're highly unlikely to get more than 1 or 2 permutation/combination questions if you're scoring in the 500s or even the low 600s. My advice is to focus on more fundamental skills such as working with equations, knowing your geometry rules, knowing your grammar rules (for SC), etc. These skills are more applicable to a broader range of problems.

That said, it would be good if you had at least a basic understanding of permutations and combinations so that if you get an easier question, you'll be able to do it. I wouldn't worry about it if you somehow are thrown a very difficult perm/combo question.