Good question! Yep, as milanander said, you should try to keep a consistent past tense when referring to any
events in the text - it doesn't matter whether they occured in Act 1 or Act 5.
However, when you're talking about
Shakespeare's message and the overall impact, you should use present tense (e.g. Macbeth kill
ed Banquo, which Shakespeare show
s is a consequences of his madness and a catalyst for his downfall).
This isn't something the assessors will explicitly mark you down for, and there are also circumstances where using a different tense would make more sense (e.g. I usually use present tense when talking about dialogue, like 'Lady Macbeth describe
s her hands as a "sorry sight... infirm of purpose.")
So long as your writing is fairly consistent and always clear, you'll be fine!