I find it is most helpful if you are calm while doing the test. If you are nervous, you may panick and make stupid mistakes. But if you are calm, you'll likely do everything right the first time.
For me, being calm meant NOT worrying or cramming at the last minute. So I'd do anything to distract myself from the exam - crosswords in the newspaper, attending an unrelated class, watching TV, etc.
So my answer is no, I don't think it is benefitial to skip your unrelated class right before the exam.