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.
However for the most part, when a worksheet is needed to help drill down a procedure, standard, or lesson, its effectiveness can and will vary.
In my 5th grade classroom, we use a math review series that's engaging and entertaining at the same time. In essence they are simply halfpage handouts with ten standards based math problems woven into a special picture or exciting scene. Remember, I want to keep the math review time quick, but effective.
Worksheets also, when used properly, provide both the students and parent / tutor immediate feedback as to the child's progress. This means they can be used to point out areas where the student needs further reinforcement.