It depends...

If she is outstanding and always the first then she may not need to. Kumon, in my opinion, is not good for the young kids especially for math.  If she is just good but not outstanding then she may need to and thus keep in pace with her peers.  If she is outstanding and you really want to develope her math talent, then a lot of other math progams like Arts of Problem Solving is much better though it's for middle school and high school kids.  I personally don't think elementary kids need extra work on math.  If she is not outstanding why do you want her spend so much time on math?  There are so many other things to do.  Find her talent and make a long term plan to develope her talent.