My wifes Medical were done in December of 2011 making them expired now, during November I contact my lawyer ( and several others ) and asked them the same question you are asking now. I was told the same thing by all, I should wait for the request. They gave several reasons why : Second medical could cause confusion, Also because the VO is aware the process takes longer then 1 year, there is a strong chance they can extend your medical expiry, Lastly this ties into the 1st reason on confusion, The medical process is constantly changing so new forms must be filled etc. When they send you a medical request they will provide you with the forms and instructions they want followed.
The reason I wanted to redo my medicals was to reduce the delay, but I was told if I did them on my own I might actually cause more delay. Who to say which is better .. I dont know. So now I wait .. I hope this decision doesn't hurt me later. So that is my experience in the matter
Goodluck to you all