To use or not to use... that is the question.
Here is the answer in Dr. A.'s room:
If a problem only requires calculation, NO! For example, 34*28.
If a problem requires mathematical thinking, YES!
If the problem doesn't tell you what keys to hit, then use a calculator to speed up your thinking.
For example, word problems, finding the missing angle measure, calculating a discount, converting feet to inches, etc.
However, don't stop thinking once you pick up the calculator. Always check that your answer makes sense. Just because the calculator displays an answer, doesn't mean the answer is correct!