In general it's risky to delay doing medical due to:
1. Possibility a visa officer in CPC-Mississauga could view app as incomplete and return it
2. Possibility that app will go on-hold for a set amount of time when visa office requests medical
3. Possibility that communication from CIC requesting you to do medical, will go to a wrong email address or for some reason not received properly. In general you want to limit the amount of times CIC needs to request info from you.