I would say that if you do a few questions that aren't extremely difficult, it might help get you in the groove, but if you encounter really hard ones, you could freak yourself out and kill your confidence.
If you've already truly learned the math formulas, grammar rules, etc. before your test day, then it wouldn't hurt to review a cheat sheet as a reminder. I have a photographic memory so if I do that, I'll remember how things looked on the page even if I'm having trouble remembering how to solve the problem. But if I don't really know the material, I can easily confuse myself by doing that. I easily got As on all my psych quizzes in college by memorizing the review pages for each chapter. LOL.
On a related note...as far as your diet goes that day, eat a higher protein, lower carb lunch to keep yourself mentally alert. If I eat a tuna sandwich I am sharp the rest of the day. Pasta, and I need a nap. A good snack for breaks: an apple, a small banana, a granola bar, a protein bar, something like that. A mini Snickers bar is good too...a burst of protein without too much sugar.
And don't load up on caffeine if you don't normally have a lot already.