Well-used vocabulary words can give you a boost, but poorly used words can definitely hurt you. While you certainly can score a 5 (or higher!) without high-level words, provided that your writing is strong, analytical, and well structured, it's always good to demonstrate that your vocabulary is up to graduate level expectations.

HOWEVER:

Never force vocabulary words! If you're not sure of how to use a word, or whether it fits, skip it. And don't overuse difficult words--a few well placed ones are enough to let the reader know that your vocab skills are up to snuff.