Never use. As soon as you start MCAT study, use ONLY pencil and paper or brain.
You would not believe the number of MCAT students I have taught, who have extreme trouble doing basic arithmetic without a calculator. It causes people major problems on the exam. If this is you, start now: do all calculations in your head or on paper. If you can't do calculations without a calculator, then find an experienced math tutor and get him/her to teach you how. Be sure to practice with exponents, logs, etc.. EK will give you a good sense of how difficult the calculations will be on the real MCAT.