However, caution must be taken into account when review is repeatedly covered in your classroom. You do not want your students to become bored or frustrated with the repetition.
As a middle-school math teacher, I have seen many students who do understand abstract concepts, and even enjoy learning about things like Pi and the Pythagorean Theorem, but who appear to be unable to do so due to a weak grasp of math fundamentals.
For many middle-school children, math is a real challenge. It is at this age that the concepts begin to get more abstract, and work goes beyond "concrete" ideas like adding and subtracting. It's no surprise that at this age, math can become something the student avoids, or begins to dislike.
When a child learns to relate math to everyday questions, he will be great at it from the simplest addition all the way to trigonometry. To convert percentages, decimals and fractions is thus one essential skill.