Yes, it is advisable to have corrective surgery before there are serious symptoms. If untreated the MVR can enlarge the left ventricle and reduce the heart's ability to have effective contractions to pump blood. If there is surgery for the valve at this time, the heart may never recovery to good functionality.
I read an article by a renown valve sureon at the Mayo Clinic and he stated the biggest problem he sees are patients that have waited too long! If one is feeling well, it is difficult to have surgery with a risk that one may not feel so well after the operation, that may be wishful thinking, but understood as one wants to keep delaying that which in all likelihood will require intervention sometime in the future.
Thanks for your question, and if you have any further questions or comments you are welcome to respond. Take care.