Unless the interviewer went to your school and was on Law Review and just graduated a couple years earlier, the interviewer will not know whether you wrote or graded on to Law Review. Each school uses different standards for acceptance onto the Journal, so an employer cannot generalize. From the outside looking in, they just see Law Review. On the other hand, if you are only in the top 70% of your class but are still on Law Review, the employer will realize that you wrote on if s/he thinks hard about it. But, it is extremely rare for someone that writes well enough to get one of those coveted pure write-on spots to also be in the bottom half of the class. More often, the person who writes on is also in the top 10% or something.
Bottom line - Law Review is huge to have on your resume, though it will not make up completely for awful grades.