It depends what the essay is on. A science essay? Yeah, probably try to be as objective as possible. But an essay on a story? Nah, be as subjective as you want. Stories are, by their very nature, subjective, and there's no reason to try and separate yourself from your opinion when discussing something so opinionated in the first place. At best you'll end up with a really boring essay that doesn't actually say anything meaningful, and at worst you'll come off as really arrogant, like you think your opinion is objective fact.